message: add invariant about number of returned links
diff: @@ -142,6 +142,8 @@ impl Tree {
  /// Append one leaf to the tree.
  ///
  /// Returns links to actual nodes that has to be persisted as the result of the append.
  + /// If completed without error, at least one link to the appended
  + /// node (with metadata provided in `new_leaf`) will be returned.
  pub fn append_leaf(&mut self, new_leaf: NodeData) -> Result<Vec<EntryLink>, Error> {
  let root = self.root;
  let new_leaf_link = self.push(new_leaf.into());
  