Preparation for sharing

- rustfmt
- clippy
- comments
- README

src/utils/replace_first.rs

@@ -1,20 +1,20 @@
 use std::iter;
 
-/// Iterate over a sequence with the first element the function returns
-/// Some() for updated, but only if there is such an element.
-pub fn replace_first<
-  T: Clone,
-  F: FnMut(&T) -> Option<T>
->(
-  slice: &[T], mut f: F
+/// Iterate over a sequence with the first element updated for which the
+/// function returns Some(), but only if there is such an element.
+pub fn replace_first<T: Clone, F: FnMut(&T) -> Option<T>>(
+  slice: &[T],
+  mut f: F,
 ) -> Option<impl Iterator<Item = T> + '_> {
   for i in 0..slice.len() {
     if let Some(new) = f(&slice[i]) {
-      let subbed_iter = slice[0..i].iter().cloned()
+      let subbed_iter = slice[0..i]
+        .iter()
+        .cloned()
         .chain(iter::once(new))
-        .chain(slice[i+1..].iter().cloned());
-      return Some(subbed_iter)
+        .chain(slice[i + 1..].iter().cloned());
+      return Some(subbed_iter);
     }
   }
   None
-}
+}