diff --git a/Cargo.lock b/Cargo.lock
index fc42f65..1683f6a 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4,11 +4,12 @@ version = 3
 
 [[package]]
 name = "ahash"
-version = "0.8.9"
+version = "0.8.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d713b3834d76b85304d4d525563c1276e2e30dc97cc67bfb4585a4a29fc2c89f"
+checksum = "e89da841a80418a9b391ebaea17f5c112ffaaa96f621d2c285b5174da76b9011"
 dependencies = [
  "cfg-if",
+ "getrandom",
  "once_cell",
  "version_check",
  "zerocopy",
@@ -164,10 +165,21 @@ dependencies = [
 ]
 
 [[package]]
-name = "hashbrown"
-version = "0.14.3"
+name = "getrandom"
+version = "0.2.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "290f1a1d9242c78d09ce40a5e87e7554ee637af1351968159f4952f028f75604"
+checksum = "94b22e06ecb0110981051723910cbf0b5f5e09a2062dd7663334ee79a9d1286c"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "wasi",
+]
+
+[[package]]
+name = "hashbrown"
+version = "0.14.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
 dependencies = [
  "ahash",
  "allocator-api2",
@@ -273,14 +285,28 @@ dependencies = [
 name = "orchid-extension"
 version = "0.1.0"
 dependencies = [
+ "ahash",
+ "hashbrown",
+ "itertools",
  "orchid-api",
  "orchid-api-traits",
  "orchid-base",
+ "ordered-float",
 ]
 
 [[package]]
 name = "orchid-host"
 version = "0.1.0"
+dependencies = [
+ "derive_destructure",
+ "hashbrown",
+ "itertools",
+ "lazy_static",
+ "orchid-api",
+ "orchid-api-traits",
+ "orchid-base",
+ "ordered-float",
+]
 
 [[package]]
 name = "orchid-std"
@@ -498,6 +524,12 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "wasi"
+version = "0.11.0+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"
diff --git a/orchid-api-traits/src/coding.rs b/orchid-api-traits/src/coding.rs
index 6e2c2b5..0ab623a 100644
--- a/orchid-api-traits/src/coding.rs
+++ b/orchid-api-traits/src/coding.rs
@@ -2,7 +2,7 @@ use std::collections::HashMap;
 use std::hash::Hash;
 use std::io::{Read, Write};
 use std::iter;
-use std::ops::Range;
+use std::ops::{Range, RangeInclusive};
 use std::rc::Rc;
 use std::sync::Arc;
 
@@ -24,8 +24,8 @@ pub trait Encode {
     vec
   }
 }
-pub trait Coding: Encode + Decode {}
-impl<T: Encode + Decode> Coding for T {}
+pub trait Coding: Encode + Decode + Clone {}
+impl<T: Encode + Decode + Clone> Coding for T {}
 
 macro_rules! num_impl {
   ($number:ty, $size:expr) => {
@@ -81,7 +81,7 @@ nonzero_impl!(std::num::NonZeroI32);
 nonzero_impl!(std::num::NonZeroI64);
 nonzero_impl!(std::num::NonZeroI128);
 
-impl<'a, T: Encode> Encode for &'a T {
+impl<'a, T: Encode + 'a> Encode for &'a T {
   fn encode<W: Write>(&self, write: &mut W) { (**self).encode(write) }
 }
 impl<T: Decode + FloatCore> Decode for NotNan<T> {
@@ -233,16 +233,25 @@ impl<T: Decode, const N: usize> Decode for [T; N] {
 impl<T: Encode, const N: usize> Encode for [T; N] {
   fn encode<W: Write>(&self, write: &mut W) { self.iter().for_each(|t| t.encode(write)) }
 }
-impl<T: Decode> Decode for Range<T> {
-  fn decode<R: Read>(read: &mut R) -> Self { T::decode(read)..T::decode(read) }
-}
-impl<T: Encode> Encode for Range<T> {
-  fn encode<W: Write>(&self, write: &mut W) {
-    self.start.encode(write);
-    self.end.encode(write);
+
+macro_rules! two_end_range {
+  ($this:ident, $name:tt, $op:tt, $start:expr, $end:expr) => {
+    impl<T: Decode> Decode for $name<T> {
+      fn decode<R: Read>(read: &mut R) -> Self { T::decode(read) $op T::decode(read) }
+    }
+    impl<T: Encode> Encode for $name<T> {
+      fn encode<W: Write>(&self, write: &mut W) {
+        let $this = self;
+        ($start).encode(write);
+        ($end).encode(write);
+      }
+    }
   }
 }
 
+two_end_range!(x, Range, .., x.start, x.end);
+two_end_range!(x, RangeInclusive, ..=, x.start(), x.end());
+
 macro_rules! smart_ptr {
   ($name:tt) => {
     impl<T: Decode> Decode for $name<T> {
diff --git a/orchid-api-traits/src/lib.rs b/orchid-api-traits/src/lib.rs
index 7b7d539..be92cf0 100644
--- a/orchid-api-traits/src/lib.rs
+++ b/orchid-api-traits/src/lib.rs
@@ -9,4 +9,4 @@ pub use hierarchy::{
   Base, Extends, HierarchyRole, InHierarchy, Subtype, TLBool, TLFalse, TLTrue, UnderRoot,
   UnderRootImpl,
 };
-pub use relations::{MsgSet, Request};
+pub use relations::{Channel, MsgSet, Request};
diff --git a/orchid-api-traits/src/relations.rs b/orchid-api-traits/src/relations.rs
index b50595f..f311ded 100644
--- a/orchid-api-traits/src/relations.rs
+++ b/orchid-api-traits/src/relations.rs
@@ -5,9 +5,12 @@ pub trait Request: Coding + Sized + Send + 'static {
   fn respond(&self, rep: Self::Response) -> Vec<u8> { rep.enc_vec() }
 }
 
-pub trait MsgSet {
-  type InReq: Coding + Sized + Send + 'static;
-  type InNot: Coding + Sized + Send + 'static;
-  type OutReq: Coding + Sized + Send + 'static;
-  type OutNot: Coding + Sized + Send + 'static;
+pub trait Channel: 'static {
+  type Req: Coding + Sized + Send + 'static;
+  type Notif: Coding + Sized + Send + 'static;
+}
+
+pub trait MsgSet {
+  type In: Channel;
+  type Out: Channel;
 }
diff --git a/orchid-api/src/parser.rs b/orchid-api/src/parser.rs
index fbff40a..9ec4ff8 100644
--- a/orchid-api/src/parser.rs
+++ b/orchid-api/src/parser.rs
@@ -1,3 +1,5 @@
+use std::ops::RangeInclusive;
+
 use orchid_api_derive::{Coding, Hierarchy};
 use orchid_api_traits::Request;
 
@@ -16,11 +18,18 @@ pub enum ParserReq {
 
 pub type LexerId = u16;
 
+#[derive(Clone, Debug, Coding)]
+pub struct Lexer {
+  id: LexerId,
+  drop: bool,
+  notify_chars: Option<Vec<RangeInclusive<char>>>,
+}
+
 #[derive(Clone, Debug, Coding, Hierarchy)]
 #[extends(ParserReq, HostExtReq)]
 pub struct MkLexer(pub SysId, pub TStr);
 impl Request for MkLexer {
-  type Response = LexerId;
+  type Response = Lexer;
 }
 
 #[derive(Clone, Debug, Coding, Hierarchy)]
@@ -50,6 +59,7 @@ impl Request for SubLex {
   type Response = SubLex;
 }
 
-pub struct ParseLine {
+#[derive(Clone, Debug, Coding)]
+pub struct LexerDrop;
 
-}
+pub struct ParseLine {}
diff --git a/orchid-api/src/proto.rs b/orchid-api/src/proto.rs
index 236d7e4..e663f3f 100644
--- a/orchid-api/src/proto.rs
+++ b/orchid-api/src/proto.rs
@@ -26,7 +26,7 @@ use std::io::{Read, Write};
 
 use derive_more::{From, TryInto};
 use orchid_api_derive::{Coding, Hierarchy};
-use orchid_api_traits::{read_exact, write_exact, Decode, Encode, MsgSet, Request};
+use orchid_api_traits::{read_exact, write_exact, Channel, Decode, Encode, MsgSet, Request};
 
 use crate::{atom, expr, intern, parser, system, tree};
 
@@ -77,12 +77,18 @@ pub enum ExtHostReq {
 }
 
 /// Notifications sent from the extension to the host
-#[derive(Coding, From, TryInto)]
+#[derive(Debug, Clone, Coding, From, TryInto)]
 #[allow(clippy::enum_variant_names)]
 pub enum ExtHostNotif {
   Expr(expr::ExprNotif),
 }
 
+pub struct ExtHostChannel;
+impl Channel for ExtHostChannel {
+  type Notif = ExtHostNotif;
+  type Req = ExtHostReq;
+}
+
 /// Requests running from the host to the extension
 #[derive(Clone, Debug, Coding, Hierarchy)]
 #[extendable]
@@ -101,25 +107,28 @@ pub enum HostExtReq {
 pub enum HostExtNotif {
   SystemDrop(system::SystemDrop),
   AtomDrop(atom::AtomDrop),
+  LexerDrop(parser::LexerDrop),
   /// The host can assume that after this notif is sent, a correctly written
   /// extension will eventually exit.
   Exit,
 }
 
+pub struct HostExtChannel;
+impl Channel for HostExtChannel {
+  type Notif = HostExtNotif;
+  type Req = HostExtReq;
+}
+
 /// Message set viewed from the extension's perspective
 pub struct ExtMsgSet;
 impl MsgSet for ExtMsgSet {
-  type InNot = HostExtNotif;
-  type InReq = HostExtReq;
-  type OutNot = ExtHostNotif;
-  type OutReq = ExtHostReq;
+  type In = HostExtChannel;
+  type Out = ExtHostChannel;
 }
 
 /// Message Set viewed from the host's perspective
 pub struct HostMsgSet;
 impl MsgSet for HostMsgSet {
-  type InNot = ExtHostNotif;
-  type InReq = ExtHostReq;
-  type OutNot = HostExtNotif;
-  type OutReq = HostExtReq;
+  type In = ExtHostChannel;
+  type Out = HostExtChannel;
 }
diff --git a/orchid-api/src/system.rs b/orchid-api/src/system.rs
index b29e1d1..79cc070 100644
--- a/orchid-api/src/system.rs
+++ b/orchid-api/src/system.rs
@@ -11,7 +11,7 @@ pub type SysDeclId = u16;
 pub type SysId = u16;
 
 /// Details about a system provided by this library
-#[derive(Coding)]
+#[derive(Debug, Clone, Coding)]
 pub struct SystemDecl {
   /// ID of the system, unique within the library
   pub id: SysDeclId,
diff --git a/orchid-base/src/gen/impls.rs b/orchid-base/src/gen/impls.rs
index 9793130..2b420ee 100644
--- a/orchid-base/src/gen/impls.rs
+++ b/orchid-base/src/gen/impls.rs
@@ -5,8 +5,6 @@ use orchid_api::expr::{Clause, Expr};
 use orchid_api::location::Location;
 
 use super::traits::{GenClause, Generable};
-use crate::expr::RtExpr;
-use crate::host::AtomHand;
 use crate::intern::{deintern, intern};
 
 fn safely_reinterpret<In: 'static, Out: 'static>(x: In) -> Result<Out, In> {
@@ -24,7 +22,7 @@ impl GenClause for Expr {
   fn generate<T: super::traits::Generable>(&self, ctx: T::Ctx<'_>, pop: &impl Fn() -> T) -> T {
     match &self.clause {
       Clause::Arg(arg) => T::arg(ctx, deintern(*arg).as_str()),
-      Clause::Atom(atom) => T::atom(ctx, AtomHand::from_api(atom.clone())),
+      Clause::Atom(atom) => T::atom(ctx, atom.clone()),
       Clause::Call(f, x) => T::apply(ctx, |c| f.generate(c, pop), |c| x.generate(c, pop)),
       Clause::Lambda(arg, b) => T::lambda(ctx, deintern(*arg).as_str(), |ctx| b.generate(ctx, pop)),
       Clause::Seq(n1, n2) => T::seq(ctx, |c| n1.generate(c, pop), |c| n2.generate(c, pop)),
diff --git a/orchid-base/src/gen/tpl.rs b/orchid-base/src/gen/tpl.rs
index 69c362b..f063f28 100644
--- a/orchid-base/src/gen/tpl.rs
+++ b/orchid-base/src/gen/tpl.rs
@@ -1,12 +1,13 @@
 //! Various elemental components to build expression trees that all implement
 //! [GenClause].
 
+use orchid_api::atom::Atom;
+
 use super::traits::{GenClause, Generable};
-use crate::host::AtomHand;
 
 /// A trivial atom
 #[derive(Clone, Debug)]
-pub struct SysAtom(pub AtomHand);
+pub struct SysAtom(pub Atom);
 impl GenClause for SysAtom {
   fn generate<T: Generable>(&self, ctx: T::Ctx<'_>, _: &impl Fn() -> T) -> T {
     T::atom(ctx, self.0.clone())
diff --git a/orchid-base/src/gen/traits.rs b/orchid-base/src/gen/traits.rs
index bbb33c7..668270b 100644
--- a/orchid-base/src/gen/traits.rs
+++ b/orchid-base/src/gen/traits.rs
@@ -5,7 +5,7 @@ use std::cell::RefCell;
 use std::collections::VecDeque;
 use std::fmt;
 
-use crate::host::AtomHand;
+use orchid_api::atom::Atom;
 
 /// Representations of the Orchid expression tree that can describe basic
 /// language elements.
@@ -13,7 +13,7 @@ pub trait Generable: Sized + 'static {
   /// Context information defined by parents. Generators just forward this.
   type Ctx<'a>: Sized;
   /// Wrap external data.
-  fn atom(ctx: Self::Ctx<'_>, a: AtomHand) -> Self;
+  fn atom(ctx: Self::Ctx<'_>, a: Atom) -> Self;
   /// Generate a reference to a constant
   fn constant<'a>(ctx: Self::Ctx<'_>, name: impl IntoIterator<Item = &'a str>) -> Self;
   /// Generate a function call given the function and its argument
diff --git a/orchid-base/src/gen/tree.rs b/orchid-base/src/gen/tree.rs
index a2aa3cd..9451cce 100644
--- a/orchid-base/src/gen/tree.rs
+++ b/orchid-base/src/gen/tree.rs
@@ -4,13 +4,13 @@
 use std::fmt;
 
 use dyn_clone::{clone_box, DynClone};
+use orchid_api::atom::Atom;
 use orchid_api::expr::Expr;
 use trait_set::trait_set;
 
 use super::tpl;
 use super::traits::{Gen, GenClause};
 use crate::combine::Combine;
-use crate::host::AtomHand;
 use crate::tree::{ModEntry, ModMember, TreeConflict};
 
 trait_set! {
@@ -64,14 +64,14 @@ pub fn ent<K: AsRef<str>>(
 
 /// Describe an [Atomic]
 #[must_use]
-pub fn atom_leaf(atom: AtomHand) -> ConstTree { leaf(tpl::SysAtom(atom)) }
+pub fn atom_leaf(atom: Atom) -> ConstTree { leaf(tpl::SysAtom(atom)) }
 
 /// Describe an [Atomic] which appears as an entry in a [ConstTree::tree]
 ///
 /// The unarray is used to trick rustfmt into breaking the atom into a block
 /// without breaking this call into a block
 #[must_use]
-pub fn atom_ent<K: AsRef<str>>(key: K, [atom]: [AtomHand; 1]) -> (K, ConstTree) {
+pub fn atom_ent<K: AsRef<str>>(key: K, [atom]: [Atom; 1]) -> (K, ConstTree) {
   (key, atom_leaf(atom))
 }
 
diff --git a/orchid-base/src/lib.rs b/orchid-base/src/lib.rs
index 3765909..8784840 100644
--- a/orchid-base/src/lib.rs
+++ b/orchid-base/src/lib.rs
@@ -1,10 +1,9 @@
 pub mod boxed_iter;
-pub mod child;
+pub mod msg;
 pub mod clone;
 pub mod combine;
 pub mod event;
-pub mod expr;
-pub mod gen;
+// pub mod gen;
 pub mod intern;
 pub mod location;
 pub mod name;
diff --git a/orchid-base/src/msg.rs b/orchid-base/src/msg.rs
new file mode 100644
index 0000000..85c29c3
--- /dev/null
+++ b/orchid-base/src/msg.rs
@@ -0,0 +1,16 @@
+use std::io;
+
+pub fn send_msg(write: &mut impl io::Write, msg: &[u8]) -> io::Result<()> {
+  write.write_all(&(u32::try_from(msg.len()).unwrap()).to_be_bytes())?;
+  write.write_all(msg)?;
+  write.flush()
+}
+
+pub fn recv_msg(read: &mut impl io::Read) -> io::Result<Vec<u8>> {
+  let mut len = [0u8; 4];
+  read.read_exact(&mut len)?;
+  let len = u32::from_be_bytes(len);
+  let mut msg = vec![0u8; len as usize];
+  read.read_exact(&mut msg)?;
+  Ok(msg)
+}
diff --git a/orchid-base/src/reqnot.rs b/orchid-base/src/reqnot.rs
index ff360a7..5b57539 100644
--- a/orchid-base/src/reqnot.rs
+++ b/orchid-base/src/reqnot.rs
@@ -6,13 +6,13 @@ use std::sync::{Arc, Mutex};
 
 use dyn_clone::{clone_box, DynClone};
 use hashbrown::HashMap;
-use orchid_api_traits::{Coding, Decode, Encode, MsgSet, Request};
+use orchid_api_traits::{Channel, Coding, Decode, Encode, MsgSet, Request};
 use trait_set::trait_set;
 
 trait_set! {
   pub trait SendFn<T: MsgSet> = for<'a> FnMut(&'a [u8], ReqNot<T>) + DynClone + Send + 'static;
   pub trait ReqFn<T: MsgSet> = FnMut(RequestHandle<T>) + Send + 'static;
-  pub trait NotifFn<T: MsgSet> = for<'a> FnMut(T::InNot, ReqNot<T>) + Send + Sync + 'static;
+  pub trait NotifFn<T: MsgSet> = for<'a> FnMut(<T::In as Channel>::Notif, ReqNot<T>) + Send + Sync + 'static;
 }
 
 fn get_id(message: &[u8]) -> (u64, &[u8]) {
@@ -21,14 +21,14 @@ fn get_id(message: &[u8]) -> (u64, &[u8]) {
 
 pub struct RequestHandle<T: MsgSet> {
   id: u64,
-  message: T::InReq,
+  message: <T::In as Channel>::Req,
   send: Box<dyn SendFn<T>>,
   parent: ReqNot<T>,
   fulfilled: AtomicBool,
 }
 impl<MS: MsgSet> RequestHandle<MS> {
   pub fn reqnot(&self) -> ReqNot<MS> { self.parent.clone() }
-  pub fn req(&self) -> &MS::InReq { &self.message }
+  pub fn req(&self) -> &<MS::In as Channel>::Req { &self.message }
   fn respond(&self, response: &impl Encode) {
     assert!(!self.fulfilled.swap(true, Ordering::Relaxed), "Already responded");
     let mut buf = (!self.id).to_be_bytes().to_vec();
@@ -78,21 +78,21 @@ impl<T: MsgSet> ReqNot<T> {
     let mut g = self.0.lock().unwrap();
     let (id, payload) = get_id(&message[..]);
     if id == 0 {
-      (g.notif)(T::InNot::decode(&mut &payload[..]), self.clone())
+      (g.notif)(<T::In as Channel>::Notif::decode(&mut &payload[..]), self.clone())
     } else if 0 < id.bitand(1 << 63) {
       let sender = g.responses.remove(&!id).expect("Received response for invalid message");
       sender.send(message).unwrap();
     } else {
       let send = clone_box(&*g.send);
-      let message = T::InReq::decode(&mut &payload[..]);
+      let message = <T::In as Channel>::Req::decode(&mut &payload[..]);
       (g.req)(RequestHandle { id, message, send, fulfilled: false.into(), parent: self.clone() })
     }
   }
 
-  pub fn notify<N: Coding + Into<T::OutNot>>(&self, notif: N) {
+  pub fn notify<N: Coding + Into<<T::Out as Channel>::Notif>>(&self, notif: N) {
     let mut send = clone_box(&*self.0.lock().unwrap().send);
     let mut buf = vec![0; 8];
-    let msg: T::OutNot = notif.into();
+    let msg: <T::Out as Channel>::Notif = notif.into();
     msg.encode(&mut buf);
     send(&buf, self.clone())
   }
@@ -112,7 +112,7 @@ impl<'a, T> DynRequester for MappedRequester<'a, T> {
 }
 
 impl<T: MsgSet> DynRequester for ReqNot<T> {
-  type Transfer = T::OutReq;
+  type Transfer = <T::Out as Channel>::Req;
   fn raw_request(&self, req: Self::Transfer) -> RawReply {
     let mut g = self.0.lock().unwrap();
     let id = g.id;
@@ -156,23 +156,27 @@ mod test {
   use std::sync::{Arc, Mutex};
 
   use orchid_api_derive::Coding;
-  use orchid_api_traits::Request;
+  use orchid_api_traits::{Channel, Request};
 
   use super::{MsgSet, ReqNot};
   use crate::{clone, reqnot::Requester as _};
 
-  #[derive(Coding, Debug, PartialEq)]
+  #[derive(Clone, Debug, Coding, PartialEq)]
   pub struct TestReq(u8);
   impl Request for TestReq {
     type Response = u8;
   }
 
+  pub struct TestChan;
+  impl Channel for TestChan {
+    type Notif = u8;
+    type Req = TestReq;
+  }
+
   pub struct TestMsgSet;
   impl MsgSet for TestMsgSet {
-    type InNot = u8;
-    type InReq = TestReq;
-    type OutNot = u8;
-    type OutReq = TestReq;
+    type In = TestChan;
+    type Out = TestChan;
   }
 
   #[test]
diff --git a/orchid-extension/Cargo.toml b/orchid-extension/Cargo.toml
index e8fda3d..d5c24a6 100644
--- a/orchid-extension/Cargo.toml
+++ b/orchid-extension/Cargo.toml
@@ -6,6 +6,10 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+ahash = "0.8.11"
+hashbrown = "0.14.5"
+itertools = "0.12.1"
 orchid-api = { version = "0.1.0", path = "../orchid-api" }
 orchid-api-traits = { version = "0.1.0", path = "../orchid-api-traits" }
 orchid-base = { version = "0.1.0", path = "../orchid-base" }
+ordered-float = "4.2.0"
diff --git a/orchid-extension/src/data.rs b/orchid-extension/src/data.rs
new file mode 100644
index 0000000..04a442d
--- /dev/null
+++ b/orchid-extension/src/data.rs
@@ -0,0 +1,137 @@
+use std::any::TypeId;
+use std::hash::{Hash, Hasher};
+use std::marker::PhantomData;
+
+use itertools::Itertools;
+use orchid_api::expr::Expr;
+use orchid_api::proto::ExtMsgSet;
+use orchid_api::system::{NewSystem, SysId, SystemDecl};
+use orchid_api_traits::Coding;
+use orchid_base::reqnot::ReqNot;
+use ordered_float::NotNan;
+
+pub struct SystemHandle<T: SystemDepCard> {
+  _t: PhantomData<T>,
+  id: SysId,
+  reqnot: ReqNot<ExtMsgSet>,
+}
+impl<T: SystemDepCard> SystemHandle<T> {
+  fn new(id: SysId, reqnot: ReqNot<ExtMsgSet>) -> Self { Self { _t: PhantomData, id, reqnot } }
+}
+
+pub trait System: Send {
+  fn consts(&self) -> Expr;
+}
+
+pub struct SystemParams<Ctor: SystemCtor + ?Sized> {
+  pub deps: <Ctor::Deps as DepSet>::Sat,
+  pub id: SysId,
+  pub reqnot: ReqNot<ExtMsgSet>,
+}
+
+pub trait SystemDepCard {
+  type IngressReq: Coding;
+  type IngressNotif: Coding;
+  const NAME: &'static str;
+}
+
+pub trait DepSet {
+  type Sat;
+  fn report(names: &mut impl FnMut(&'static str));
+  fn create(take: &mut impl FnMut() -> SysId, reqnot: ReqNot<ExtMsgSet>) -> Self::Sat;
+}
+
+impl<T: SystemDepCard> DepSet for T {
+  type Sat = SystemHandle<Self>;
+  fn report(names: &mut impl FnMut(&'static str)) { names(T::NAME) }
+  fn create(take: &mut impl FnMut() -> SysId, reqnot: ReqNot<ExtMsgSet>) -> Self::Sat {
+    SystemHandle::new(take(), reqnot)
+  }
+}
+
+pub trait SystemCtor: Send {
+  type Deps: DepSet;
+  const NAME: &'static str;
+  const VERSION: f64;
+  #[allow(clippy::new_ret_no_self)]
+  fn new(params: SystemParams<Self>) -> Box<dyn System>;
+}
+
+pub trait DynSystemCtor: Send {
+  fn decl(&self) -> SystemDecl;
+  fn new_system(&self, new: &NewSystem, reqnot: ReqNot<ExtMsgSet>) -> Box<dyn System>;
+}
+
+impl<T: SystemCtor + 'static> DynSystemCtor for T {
+  fn decl(&self) -> SystemDecl {
+    // Version is equivalent to priority for all practical purposes
+    let priority = NotNan::new(T::VERSION).unwrap();
+    // aggregate depends names
+    let mut depends = Vec::new();
+    T::Deps::report(&mut |n| depends.push(n.to_string()));
+    // generate definitely unique id
+    let mut ahash = ahash::AHasher::default();
+    TypeId::of::<T>().hash(&mut ahash);
+    let id = (ahash.finish().to_be_bytes().into_iter().tuples())
+      .map(|(l, b)| u16::from_be_bytes([l, b]))
+      .fold(0, |a, b| a ^ b);
+    SystemDecl { name: T::NAME.to_string(), depends, id, priority }
+  }
+  fn new_system(&self, new: &NewSystem, reqnot: ReqNot<ExtMsgSet>) -> Box<dyn System> {
+    let mut ids = new.depends.iter().copied();
+    T::new(SystemParams {
+      deps: T::Deps::create(&mut || ids.next().unwrap(), reqnot.clone()),
+      id: new.id,
+      reqnot,
+    })
+  }
+}
+
+pub struct ExtensionData {
+  pub systems: Vec<Box<dyn DynSystemCtor>>,
+}
+
+mod dep_set_tuple_impls {
+  use orchid_api::proto::ExtMsgSet;
+  use orchid_api::system::SysId;
+  use orchid_base::reqnot::ReqNot;
+
+  use super::DepSet;
+
+  macro_rules! dep_set_tuple_impl {
+    ($($name:ident),*) => {
+      impl<$( $name :DepSet ),*> DepSet for ( $( $name , )* ) {
+        type Sat = ( $( $name ::Sat , )* );
+        fn report(names: &mut impl FnMut(&'static str)) {
+          $(
+            $name ::report(names);
+          )*
+        }
+        fn create(take: &mut impl FnMut() -> SysId, reqnot: ReqNot<ExtMsgSet>) -> Self::Sat {
+          (
+            $(
+              $name ::create(take, reqnot.clone()),
+            )*
+          )
+        }
+      }
+    };
+  }
+
+  dep_set_tuple_impl!(A);
+  dep_set_tuple_impl!(A, B); // 2
+  dep_set_tuple_impl!(A, B, C);
+  dep_set_tuple_impl!(A, B, C, D); // 4
+  dep_set_tuple_impl!(A, B, C, D, E);
+  dep_set_tuple_impl!(A, B, C, D, E, F);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H); // 8
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I, J);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I, J, K);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I, J, K, L); // 12
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I, J, K, L, M);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I, J, K, L, M, N);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O);
+  dep_set_tuple_impl!(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P); // 16
+}
diff --git a/orchid-extension/src/entrypoint.rs b/orchid-extension/src/entrypoint.rs
index 7cf6453..5c00708 100644
--- a/orchid-extension/src/entrypoint.rs
+++ b/orchid-extension/src/entrypoint.rs
@@ -1,19 +1,23 @@
 use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 
+use hashbrown::HashMap;
+use itertools::Itertools;
 use orchid_api::proto::{ExtMsgSet, ExtensionHeader, HostExtNotif, HostExtReq, HostHeader};
 use orchid_api_traits::{Decode, Encode};
-use orchid_base::child::{recv_parent_msg, send_parent_msg};
 use orchid_base::clone;
 use orchid_base::intern::{init_replica, sweep_replica};
 use orchid_base::reqnot::{ReqNot, Requester};
 
-pub struct ExtensionData {}
+use crate::data::ExtensionData;
+use crate::msg::{recv_parent_msg, send_parent_msg};
 
-pub fn main(data: &mut ExtensionData) {
+pub fn main(data: ExtensionData) {
   HostHeader::decode(&mut &recv_parent_msg().unwrap()[..]);
   let mut buf = Vec::new();
-  ExtensionHeader { systems: vec![] }.encode(&mut buf);
+  let decls = data.systems.iter().map(|sys| sys.decl()).collect_vec();
+  let systems = Arc::new(Mutex::new(HashMap::new()));
+  ExtensionHeader { systems: decls.clone() }.encode(&mut buf);
   send_parent_msg(&buf).unwrap();
   let exiting = Arc::new(AtomicBool::new(false));
   let rn = ReqNot::<ExtMsgSet>::new(
@@ -22,11 +26,17 @@ pub fn main(data: &mut ExtensionData) {
       HostExtNotif::Exit => exiting.store(true, Ordering::Relaxed),
       _ => todo!(),
     }),
-    |req| match req.req() {
+    clone!(systems; move |req| match req.req() {
       HostExtReq::Ping(ping) => req.handle(ping, &()),
       HostExtReq::Sweep(sweep) => req.handle(sweep, &sweep_replica()),
+      HostExtReq::NewSystem(new_sys) => {
+        let i = decls.iter().enumerate().find(|(_, s)| s.id == new_sys.system).unwrap().0;
+        let system = data.systems[i].new_system(new_sys, req.reqnot());
+        systems.lock().unwrap().insert(new_sys.id, system);
+        req.handle(new_sys, &())
+      },
       _ => todo!(),
-    },
+    }),
   );
   init_replica(rn.clone().map());
   while !exiting.load(Ordering::Relaxed) {
diff --git a/orchid-extension/src/extension.rs b/orchid-extension/src/extension.rs
deleted file mode 100644
index a1b0244..0000000
--- a/orchid-extension/src/extension.rs
+++ /dev/null
@@ -1,30 +0,0 @@
-use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::Arc;
-
-use orchid_api::proto::{ExtMsgSet, ExtensionHeader, HostExtNotif, HostExtReq, HostHeader};
-use orchid_api_traits::{Decode, Encode};
-use ordered_float::NotNan;
-
-use crate::child::{recv_parent_msg, send_parent_msg};
-use crate::clone;
-use crate::intern::{init_replica, sweep_replica};
-use crate::reqnot::{ReqNot, Requester as _};
-
-pub struct SystemParams {
-  deps: Vec<SystemHandle>,
-  
-}
-
-pub struct SystemCtor {
-  deps: Vec<String>,
-  make: Box<dyn FnMut(SystemParams) -> System>,
-  name: String,
-  prio: NotNan<f64>,
-  dependencies: Vec<String>,
-}
-
-pub struct ExtensionData {
-  systems: Vec<SystemCtor>
-}
-
-
diff --git a/orchid-extension/src/lib.rs b/orchid-extension/src/lib.rs
index f5ecc5a..74a206f 100644
--- a/orchid-extension/src/lib.rs
+++ b/orchid-extension/src/lib.rs
@@ -1 +1,3 @@
 pub mod entrypoint;
+pub mod data;
+pub mod msg;
diff --git a/orchid-extension/src/msg.rs b/orchid-extension/src/msg.rs
new file mode 100644
index 0000000..05fa165
--- /dev/null
+++ b/orchid-extension/src/msg.rs
@@ -0,0 +1,6 @@
+use std::io;
+
+use orchid_base::msg::{recv_msg, send_msg};
+
+pub fn send_parent_msg(msg: &[u8]) -> io::Result<()> { send_msg(&mut io::stdout().lock(), msg) }
+pub fn recv_parent_msg() -> io::Result<Vec<u8>> { recv_msg(&mut io::stdin().lock()) }
diff --git a/orchid-host/Cargo.toml b/orchid-host/Cargo.toml
index 1c3667f..2165415 100644
--- a/orchid-host/Cargo.toml
+++ b/orchid-host/Cargo.toml
@@ -6,3 +6,11 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+derive_destructure = "1.0.0"
+hashbrown = "0.14.5"
+itertools = "0.12.1"
+lazy_static = "1.4.0"
+orchid-api = { version = "0.1.0", path = "../orchid-api" }
+orchid-api-traits = { version = "0.1.0", path = "../orchid-api-traits" }
+orchid-base = { version = "0.1.0", path = "../orchid-base" }
+ordered-float = "4.2.0"
diff --git a/orchid-base/src/child.rs b/orchid-host/src/child.rs
similarity index 56%
rename from orchid-base/src/child.rs
rename to orchid-host/src/child.rs
index 319915c..c3d731a 100644
--- a/orchid-base/src/child.rs
+++ b/orchid-host/src/child.rs
@@ -1,7 +1,9 @@
-use std::io::{self, Read, Write};
+use std::io;
 use std::sync::Mutex;
 use std::{mem, process};
 
+use orchid_base::msg::{recv_msg, send_msg};
+
 pub struct SharedChild {
   child: process::Child,
   stdin: Mutex<process::ChildStdin>,
@@ -24,21 +26,3 @@ impl SharedChild {
 impl Drop for SharedChild {
   fn drop(&mut self) { mem::drop(self.child.kill()) }
 }
-
-pub fn send_msg(write: &mut impl Write, msg: &[u8]) -> io::Result<()> {
-  write.write_all(&(u32::try_from(msg.len()).unwrap()).to_be_bytes())?;
-  write.write_all(msg)?;
-  write.flush()
-}
-
-pub fn recv_msg(read: &mut impl Read) -> io::Result<Vec<u8>> {
-  let mut len = [0u8; 4];
-  read.read_exact(&mut len)?;
-  let len = u32::from_be_bytes(len);
-  let mut msg = vec![0u8; len as usize];
-  read.read_exact(&mut msg)?;
-  Ok(msg)
-}
-
-pub fn send_parent_msg(msg: &[u8]) -> io::Result<()> { send_msg(&mut io::stdout().lock(), msg) }
-pub fn recv_parent_msg() -> io::Result<Vec<u8>> { recv_msg(&mut io::stdin().lock()) }
diff --git a/orchid-host/src/expr.rs b/orchid-host/src/expr.rs
index 5a272c1..71e4cd3 100644
--- a/orchid-host/src/expr.rs
+++ b/orchid-host/src/expr.rs
@@ -2,10 +2,10 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, RwLock};
 
 use hashbrown::HashMap;
+use lazy_static::lazy_static;
 use orchid_api::expr::ExprTicket;
 
-use crate::host::AtomHand;
-use lazy_static::lazy_static;
+use crate::extension::AtomHand;
 
 #[derive(Clone, Debug)]
 pub struct RtExpr {
@@ -36,6 +36,6 @@ impl Drop for RtExpr {
   }
 }
 
-lazy_static!{
+lazy_static! {
   static ref KNOWN_EXPRS: RwLock<HashMap<ExprTicket, RtExpr>> = RwLock::default();
 }
diff --git a/orchid-host/src/extension.rs b/orchid-host/src/extension.rs
new file mode 100644
index 0000000..7ee0239
--- /dev/null
+++ b/orchid-host/src/extension.rs
@@ -0,0 +1,234 @@
+use std::io::Write as _;
+use std::sync::atomic::{AtomicU16, AtomicU32, Ordering};
+use std::sync::{Arc, Mutex, RwLock, Weak};
+use std::{fmt, io, process, thread};
+
+use derive_destructure::destructure;
+use hashbrown::HashMap;
+use itertools::Itertools;
+use lazy_static::lazy_static;
+use orchid_api::atom::{Atom, AtomDrop, AtomSame, CallRef, FinalCall, Fwd, Fwded};
+use orchid_api::expr::{Acquire, Expr, ExprNotif, ExprTicket, Release, Relocate};
+use orchid_api::intern::IntReq;
+use orchid_api::proto::{
+  ExtHostNotif, ExtHostReq, ExtensionHeader, HostExtNotif, HostHeader, HostMsgSet,
+};
+use orchid_api::system::{NewSystem, SysDeclId, SysId, SystemDecl, SystemDrop};
+use orchid_api::tree::{GetConstTree, TreeModule};
+use orchid_api_traits::{Decode, Encode};
+use orchid_base::clone;
+use orchid_base::intern::{deintern, intern};
+use orchid_base::reqnot::{ReqNot, Requester as _};
+use ordered_float::NotNan;
+
+use crate::expr::RtExpr;
+
+#[derive(Debug, destructure)]
+pub struct AtomData {
+  owner: System,
+  drop: bool,
+  data: Vec<u8>,
+}
+impl AtomData {
+  fn api(self) -> Atom {
+    let (owner, drop, data) = self.destructure();
+    Atom { data, drop, owner: owner.0.id }
+  }
+  fn api_ref(&self) -> Atom {
+    Atom { data: self.data.clone(), drop: self.drop, owner: self.owner.0.id }
+  }
+}
+impl Drop for AtomData {
+  fn drop(&mut self) {
+    self.owner.0.ext.0.reqnot.notify(AtomDrop(Atom {
+      owner: self.owner.0.id,
+      data: self.data.clone(),
+      drop: true,
+    }))
+  }
+}
+
+#[derive(Clone, Debug)]
+pub struct AtomHand(Arc<AtomData>);
+impl AtomHand {
+  pub fn from_api(Atom { data, drop, owner }: Atom) -> Self {
+    let owner = System::resolve(owner).expect("Atom owned by non-existing system");
+    Self(Arc::new(AtomData { data, drop, owner }))
+  }
+  pub fn call(self, arg: RtExpr) -> Expr {
+    let owner_sys = self.0.owner.clone();
+    let ext = &owner_sys.0.ext;
+    let ticket = owner_sys.give_expr(arg.canonicalize(), || arg);
+    match Arc::try_unwrap(self.0) {
+      Ok(data) => ext.0.reqnot.request(FinalCall(data.api(), ticket)),
+      Err(hand) => ext.0.reqnot.request(CallRef(hand.api_ref(), ticket)),
+    }
+  }
+  pub fn same(&self, other: &AtomHand) -> bool {
+    let owner = self.0.owner.0.id;
+    if other.0.owner.0.id != owner {
+      return false;
+    }
+    self.0.owner.0.ext.0.reqnot.request(AtomSame(self.0.api_ref(), other.0.api_ref()))
+  }
+  pub fn req(&self, req: Vec<u8>) -> Vec<u8> {
+    self.0.owner.0.ext.0.reqnot.request(Fwded(self.0.api_ref(), req))
+  }
+  pub fn api_ref(&self) -> Atom { self.0.api_ref() }
+}
+
+/// Data held about an Extension. This is refcounted within [Extension]. It's
+/// important to only ever access parts of this struct through the [Arc] because
+/// the components reference each other through [Weak]s of it, and will panic if
+/// upgrading fails.
+#[derive(destructure)]
+pub struct ExtensionData {
+  child: Mutex<process::Child>,
+  reqnot: ReqNot<HostMsgSet>,
+  systems: Vec<SystemCtor>,
+}
+impl Drop for ExtensionData {
+  fn drop(&mut self) { self.reqnot.notify(HostExtNotif::Exit) }
+}
+
+fn acq_expr(sys: SysId, extk: ExprTicket) {
+  (System::resolve(sys).expect("Expr acq'd by invalid system"))
+    .give_expr(extk, || RtExpr::resolve(extk).expect("Invalid expr acq'd"));
+}
+
+fn rel_expr(sys: SysId, extk: ExprTicket) {
+  let sys = System::resolve(sys).unwrap();
+  let mut exprs = sys.0.exprs.write().unwrap();
+  exprs.entry(extk).and_replace_entry_with(|_, (rc, rt)| {
+    (0 < rc.fetch_sub(1, Ordering::Relaxed)).then_some((rc, rt))
+  });
+}
+
+#[derive(Clone)]
+pub struct Extension(Arc<ExtensionData>);
+impl Extension {
+  pub fn new(mut cmd: process::Command) -> io::Result<Self> {
+    let mut child = cmd.stdin(process::Stdio::piped()).stdout(process::Stdio::piped()).spawn()?;
+    HostHeader.encode(child.stdin.as_mut().unwrap());
+    let eh = ExtensionHeader::decode(child.stdout.as_mut().unwrap());
+    Ok(Self(Arc::new_cyclic(|weak| ExtensionData {
+      child: Mutex::new(child),
+      reqnot: ReqNot::new(
+        clone!(weak; move |sfn, _| {
+          let arc: Arc<ExtensionData> = weak.upgrade().unwrap();
+          let mut g = arc.child.lock().unwrap();
+          g.stdin.as_mut().unwrap().write_all(sfn).unwrap();
+        }),
+        |notif, _| match notif {
+          ExtHostNotif::Expr(expr) => match expr {
+            ExprNotif::Acquire(Acquire(sys, extk)) => acq_expr(sys, extk),
+            ExprNotif::Release(Release(sys, extk)) => rel_expr(sys, extk),
+            ExprNotif::Relocate(Relocate { inc, dec, expr }) => {
+              acq_expr(inc, expr);
+              rel_expr(dec, expr);
+            },
+          },
+        },
+        |req| match req.req() {
+          ExtHostReq::Ping(ping) => req.handle(ping, &()),
+          ExtHostReq::IntReq(intreq) => match intreq {
+            IntReq::InternStr(s) => req.handle(s, &intern(&**s.0).marker()),
+            IntReq::InternStrv(v) => req.handle(v, &intern(&*v.0).marker()),
+            IntReq::ExternStr(si) => req.handle(si, &deintern(si.0).arc()),
+            IntReq::ExternStrv(vi) =>
+              req.handle(vi, &Arc::new(deintern(vi.0).iter().map(|t| t.marker()).collect_vec())),
+          },
+          ExtHostReq::Fwd(fw @ Fwd(atom, _body)) => {
+            let sys = System::resolve(atom.owner).unwrap();
+            thread::spawn(clone!(fw; move || {
+              req.handle(&fw, &sys.0.ext.0.reqnot.request(Fwded(fw.0.clone(), fw.1.clone())))
+            }));
+          },
+          _ => todo!(),
+        },
+      ),
+      systems: eh.systems.into_iter().map(|decl| SystemCtor { decl, ext: weak.clone() }).collect(),
+    })))
+  }
+  pub fn systems(&self) -> impl Iterator<Item = &SystemCtor> { self.0.systems.iter() }
+}
+
+pub struct SystemCtor {
+  decl: SystemDecl,
+  ext: Weak<ExtensionData>,
+}
+impl SystemCtor {
+  pub fn name(&self) -> &str { &self.decl.name }
+  pub fn priority(&self) -> NotNan<f64> { self.decl.priority }
+  pub fn depends(&self) -> impl ExactSizeIterator<Item = &str> {
+    self.decl.depends.iter().map(|s| &**s)
+  }
+  pub fn run<'a>(&self, depends: impl IntoIterator<Item = &'a System>) -> System {
+    let mut inst_g = SYSTEM_INSTS.write().unwrap();
+    let depends = depends.into_iter().map(|si| si.0.id).collect_vec();
+    debug_assert_eq!(depends.len(), self.decl.depends.len(), "Wrong number of deps provided");
+    let ext = self.ext.upgrade().expect("SystemCtor should be freed before Extension");
+    static NEXT_ID: AtomicU16 = AtomicU16::new(0);
+    let id = NEXT_ID.fetch_add(1, Ordering::Relaxed);
+    let () = ext.reqnot.request(NewSystem { depends, id, system: self.decl.id });
+    let data = System(Arc::new(SystemInstData {
+      decl_id: self.decl.id,
+      ext: Extension(ext),
+      exprs: RwLock::default(),
+      id,
+    }));
+    inst_g.insert(id, data.clone());
+    data
+  }
+}
+
+lazy_static! {
+  static ref SYSTEM_INSTS: RwLock<HashMap<u16, System>> = RwLock::default();
+}
+
+#[derive(destructure)]
+pub struct SystemInstData {
+  exprs: RwLock<HashMap<ExprTicket, (AtomicU32, RtExpr)>>,
+  ext: Extension,
+  decl_id: SysDeclId,
+  id: u16,
+}
+impl Drop for SystemInstData {
+  fn drop(&mut self) {
+    self.ext.0.reqnot.notify(SystemDrop(self.id));
+    if let Ok(mut g) = SYSTEM_INSTS.write() {
+      g.remove(&self.id);
+    }
+  }
+}
+#[derive(Clone)]
+pub struct System(Arc<SystemInstData>);
+impl System {
+  fn resolve(id: u16) -> Option<System> { SYSTEM_INSTS.read().unwrap().get(&id).cloned() }
+  fn give_expr(&self, ticket: ExprTicket, get_expr: impl FnOnce() -> RtExpr) -> ExprTicket {
+    let mut exprs = self.0.exprs.write().unwrap();
+    exprs
+      .entry(ticket)
+      .and_modify(|(c, _)| {
+        c.fetch_add(1, Ordering::Relaxed);
+      })
+      .or_insert((AtomicU32::new(1), get_expr()));
+    ticket
+  }
+
+  pub fn const_tree(&self) -> TreeModule { self.0.ext.0.reqnot.request(GetConstTree(self.0.id)) }
+}
+impl fmt::Debug for System {
+  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    let ctor = (self.0.ext.0.systems.iter().find(|c| c.decl.id == self.0.decl_id))
+      .expect("System instance with no associated constructor");
+    write!(f, "System({} @ {} #{}, ", ctor.decl.name, ctor.decl.priority, self.0.id)?;
+    match self.0.exprs.read() {
+      Err(_) => write!(f, "expressions unavailable"),
+      Ok(r) => {
+        let rc: u32 = r.values().map(|v| v.0.load(Ordering::Relaxed)).sum();
+        write!(f, "{rc} refs to {} exprs", r.len())
+      },
+    }
+  }
+}
diff --git a/orchid-host/src/lib.rs b/orchid-host/src/lib.rs
index 1a9c018..a706912 100644
--- a/orchid-host/src/lib.rs
+++ b/orchid-host/src/lib.rs
@@ -1,234 +1,3 @@
-use std::io::Write as _;
-use std::sync::atomic::{AtomicU16, AtomicU32, Ordering};
-use std::sync::{Arc, Mutex, RwLock, Weak};
-use std::{fmt, io, process, thread};
-
-use derive_destructure::destructure;
-use hashbrown::HashMap;
-use itertools::Itertools;
-use lazy_static::lazy_static;
-use orchid_api::atom::{Atom, AtomDrop, AtomSame, CallRef, FinalCall, Fwd, Fwded};
-use orchid_api::expr::{Acquire, Expr, ExprNotif, ExprTicket, Release, Relocate};
-use orchid_api::intern::IntReq;
-use orchid_api::proto::{
-  ExtHostNotif, ExtHostReq, ExtensionHeader, HostExtNotif, HostHeader, HostMsgSet,
-};
-use orchid_api::system::{NewSystem, SysDeclId, SysId, SystemDecl, SystemDrop};
-use orchid_api::tree::{GetConstTree, TreeModule};
-use orchid_api_traits::{Decode, Encode};
-use ordered_float::NotNan;
-
-use crate::clone;
-use crate::expr::RtExpr;
-use crate::intern::{deintern, intern};
-use crate::reqnot::{ReqNot, Requester as _};
-
-#[derive(Debug, destructure)]
-pub struct AtomData {
-  owner: System,
-  drop: bool,
-  data: Vec<u8>,
-}
-impl AtomData {
-  fn api(self) -> Atom {
-    let (owner, drop, data) = self.destructure();
-    Atom { data, drop, owner: owner.0.id }
-  }
-  fn api_ref(&self) -> Atom {
-    Atom { data: self.data.clone(), drop: self.drop, owner: self.owner.0.id }
-  }
-}
-impl Drop for AtomData {
-  fn drop(&mut self) {
-    self.owner.0.ext.0.reqnot.notify(AtomDrop(Atom {
-      owner: self.owner.0.id,
-      data: self.data.clone(),
-      drop: true,
-    }))
-  }
-}
-
-#[derive(Clone, Debug)]
-pub struct AtomHand(Arc<AtomData>);
-impl AtomHand {
-  pub fn from_api(Atom { data, drop, owner }: Atom) -> Self {
-    let owner = System::resolve(owner).expect("Atom owned by non-existing system");
-    Self(Arc::new(AtomData { data, drop, owner }))
-  }
-  pub fn call(self, arg: RtExpr) -> Expr {
-    let owner_sys = self.0.owner.clone();
-    let ext = &owner_sys.0.ext;
-    let ticket = owner_sys.give_expr(arg.canonicalize(), || arg);
-    match Arc::try_unwrap(self.0) {
-      Ok(data) => ext.0.reqnot.request(FinalCall(data.api(), ticket)),
-      Err(hand) => ext.0.reqnot.request(CallRef(hand.api_ref(), ticket)),
-    }
-  }
-  pub fn same(&self, other: &AtomHand) -> bool {
-    let owner = self.0.owner.0.id;
-    if other.0.owner.0.id != owner {
-      return false;
-    }
-    self.0.owner.0.ext.0.reqnot.request(AtomSame(self.0.api_ref(), other.0.api_ref()))
-  }
-  pub fn req(&self, req: Vec<u8>) -> Vec<u8> {
-    self.0.owner.0.ext.0.reqnot.request(Fwded(self.0.api_ref(), req))
-  }
-  pub fn api_ref(&self) -> Atom { self.0.api_ref() }
-}
-
-/// Data held about an Extension. This is refcounted within [Extension]. It's
-/// important to only ever access parts of this struct through the [Arc] because
-/// the components reference each other through [Weak]s of it, and will panic if
-/// upgrading fails.
-#[derive(destructure)]
-pub struct ExtensionData {
-  child: Mutex<process::Child>,
-  reqnot: ReqNot<HostMsgSet>,
-  systems: Vec<SystemCtor>,
-}
-impl Drop for ExtensionData {
-  fn drop(&mut self) { self.reqnot.notify(HostExtNotif::Exit) }
-}
-
-fn acq_expr(sys: SysId, extk: ExprTicket) {
-  (System::resolve(sys).expect("Expr acq'd by invalid system"))
-    .give_expr(extk, || RtExpr::resolve(extk).expect("Invalid expr acq'd"));
-}
-
-fn rel_expr(sys: SysId, extk: ExprTicket) {
-  let sys = System::resolve(sys).unwrap();
-  let mut exprs = sys.0.exprs.write().unwrap();
-  exprs.entry(extk).and_replace_entry_with(|_, (rc, rt)| {
-    (0 < rc.fetch_sub(1, Ordering::Relaxed)).then_some((rc, rt))
-  });
-}
-
-#[derive(Clone)]
-pub struct Extension(Arc<ExtensionData>);
-impl Extension {
-  pub fn new(mut cmd: process::Command) -> io::Result<Self> {
-    let mut child = cmd.stdin(process::Stdio::piped()).stdout(process::Stdio::piped()).spawn()?;
-    HostHeader.encode(child.stdin.as_mut().unwrap());
-    let eh = ExtensionHeader::decode(child.stdout.as_mut().unwrap());
-    Ok(Self(Arc::new_cyclic(|weak| ExtensionData {
-      child: Mutex::new(child),
-      reqnot: ReqNot::new(
-        clone!(weak; move |sfn, _| {
-          let arc: Arc<ExtensionData> = weak.upgrade().unwrap();
-          let mut g = arc.child.lock().unwrap();
-          g.stdin.as_mut().unwrap().write_all(sfn).unwrap();
-        }),
-        |notif, _| match notif {
-          ExtHostNotif::Expr(expr) => match expr {
-            ExprNotif::Acquire(Acquire(sys, extk)) => acq_expr(sys, extk),
-            ExprNotif::Release(Release(sys, extk)) => rel_expr(sys, extk),
-            ExprNotif::Relocate(Relocate { inc, dec, expr }) => {
-              acq_expr(inc, expr);
-              rel_expr(dec, expr);
-            },
-          },
-        },
-        |req| match req.req() {
-          ExtHostReq::Ping(ping) => req.handle(ping, &()),
-          ExtHostReq::IntReq(intreq) => match intreq {
-            IntReq::InternStr(s) => req.handle(s, &intern(&**s.0).marker()),
-            IntReq::InternStrv(v) => req.handle(v, &intern(&*v.0).marker()),
-            IntReq::ExternStr(si) => req.handle(si, &deintern(si.0).arc()),
-            IntReq::ExternStrv(vi) =>
-              req.handle(vi, &Arc::new(deintern(vi.0).iter().map(|t| t.marker()).collect_vec())),
-          },
-          ExtHostReq::Fwd(fw @ Fwd(atom, _body)) => {
-            let sys = System::resolve(atom.owner).unwrap();
-            thread::spawn(clone!(fw; move || {
-              req.handle(&fw, &sys.0.ext.0.reqnot.request(Fwded(fw.0.clone(), fw.1.clone())))
-            }));
-          },
-          _ => todo!(),
-        },
-      ),
-      systems: eh.systems.into_iter().map(|decl| SystemCtor { decl, ext: weak.clone() }).collect(),
-    })))
-  }
-  pub fn systems(&self) -> impl Iterator<Item = &SystemCtor> { self.0.systems.iter() }
-}
-
-pub struct SystemCtor {
-  decl: SystemDecl,
-  ext: Weak<ExtensionData>,
-}
-impl SystemCtor {
-  pub fn name(&self) -> &str { &self.decl.name }
-  pub fn priority(&self) -> NotNan<f64> { self.decl.priority }
-  pub fn depends(&self) -> impl ExactSizeIterator<Item = &str> {
-    self.decl.depends.iter().map(|s| &**s)
-  }
-  pub fn run<'a>(&self, depends: impl IntoIterator<Item = &'a System>) -> System {
-    let mut inst_g = SYSTEM_INSTS.write().unwrap();
-    let depends = depends.into_iter().map(|si| si.0.id).collect_vec();
-    debug_assert_eq!(depends.len(), self.decl.depends.len(), "Wrong number of deps provided");
-    let ext = self.ext.upgrade().expect("SystemCtor should be freed before Extension");
-    static NEXT_ID: AtomicU16 = AtomicU16::new(0);
-    let id = NEXT_ID.fetch_add(1, Ordering::Relaxed);
-    let () = ext.reqnot.request(NewSystem { depends, id, system: self.decl.id });
-    let data = System(Arc::new(SystemInstData {
-      decl_id: self.decl.id,
-      ext: Extension(ext),
-      exprs: RwLock::default(),
-      id,
-    }));
-    inst_g.insert(id, data.clone());
-    data
-  }
-}
-
-lazy_static! {
-  static ref SYSTEM_INSTS: RwLock<HashMap<u16, System>> = RwLock::default();
-}
-
-#[derive(destructure)]
-pub struct SystemInstData {
-  exprs: RwLock<HashMap<ExprTicket, (AtomicU32, RtExpr)>>,
-  ext: Extension,
-  decl_id: SysDeclId,
-  id: u16,
-}
-impl Drop for SystemInstData {
-  fn drop(&mut self) {
-    self.ext.0.reqnot.notify(SystemDrop(self.id));
-    if let Ok(mut g) = SYSTEM_INSTS.write() {
-      g.remove(&self.id);
-    }
-  }
-}
-#[derive(Clone)]
-pub struct System(Arc<SystemInstData>);
-impl System {
-  fn resolve(id: u16) -> Option<System> { SYSTEM_INSTS.read().unwrap().get(&id).cloned() }
-  fn give_expr(&self, ticket: ExprTicket, get_expr: impl FnOnce() -> RtExpr) -> ExprTicket {
-    let mut exprs = self.0.exprs.write().unwrap();
-    exprs
-      .entry(ticket)
-      .and_modify(|(c, _)| {
-        c.fetch_add(1, Ordering::Relaxed);
-      })
-      .or_insert((AtomicU32::new(1), get_expr()));
-    ticket
-  }
-
-  pub fn const_tree(&self) -> TreeModule { self.0.ext.0.reqnot.request(GetConstTree(self.0.id)) }
-}
-impl fmt::Debug for System {
-  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    let ctor = (self.0.ext.0.systems.iter().find(|c| c.decl.id == self.0.decl_id))
-      .expect("System instance with no associated constructor");
-    write!(f, "System({} @ {} #{}, ", ctor.decl.name, ctor.decl.priority, self.0.id)?;
-    match self.0.exprs.read() {
-      Err(_) => write!(f, "expressions unavailable"),
-      Ok(r) => {
-        let rc: u32 = r.values().map(|v| v.0.load(Ordering::Relaxed)).sum();
-        write!(f, "{rc} refs to {} exprs", r.len())
-      },
-    }
-  }
-}
+pub mod child;
+pub mod expr;
+pub mod extension;