message: Use doc string
  Summary: as in title
diff: @@ -49,16 +49,16 @@ impl<V> ToString for Node<V> {
  }
  impl<V: Sized> Node<V> {
  - // Core algorithm for node insert, update, and removal.
  - // Returns: updated tree.
  - //
  - // `op` will be called in two separate occasions.
  - // - When node with `key` is found. Then `op` will be called with a `Some` value containing
  - // the matching node. The entire matching subtree will be replaced by return value of `op`.
  - // If `op` returned `None`, the entire subtree will be removed.
  - // - When node with `key` is not found. Then `op` will be called with a `None` value. The
  - // return value of `op` is emplaced to the tree. If the return value of `op` is `None`, a
  - // value equivalent to the original tree `maybe_node` is returned.
  + /// Core algorithm for node insert, update, and removal.
  + /// Returns: updated tree.
  + ///
  + /// `op` will be called in two separate occasions.
  + /// - When node with `key` is found. Then `op` will be called with a `Some` value containing
  + /// the matching node. The entire matching subtree will be replaced by return value of `op`.
  + /// If `op` returned `None`, the entire subtree will be removed.
  + /// - When node with `key` is not found. Then `op` will be called with a `None` value. The
  + /// return value of `op` is emplaced to the tree. If the return value of `op` is `None`, a
  + /// value equivalent to the original tree `maybe_node` is returned.
  fn update_node_by_key<F>(
  maybe_node: Option<Rc<Node<V>>>,
  key: &BitVec,
  @@ -234,9 +234,9 @@ impl<V: Sized> Node<V> {
  updated.unwrap() // Leaf combine should not make deletions.
  }
  - // Merge two trees. Combined tree should contain all keys from s and t.
  - // If duplicate keys are found, two nodes are passed to `leaf_combine` shall be called
  - // with values from s on the left hand side and values from t on the right hand side.
  + /// Merge two trees. Combined tree should contain all keys from s and t.
  + /// If duplicate keys are found, two nodes are passed to `leaf_combine` shall be called
  + /// with values from s on the left hand side and values from t on the right hand side.
  fn merge_trees(
  s: &Rc<Node<V>>,
  t: &Rc<Node<V>>,
  