code: function Leaf(edit, hash, key, value) {
      return {
          type: LEAF,
          edit: edit,
          hash: hash,
          key: key,
          value: value,
          _modify: Leaf__modify
      };
  }
docs: Leaf holding a value.
  
  @member edit Edit of the node.
  @member hash Hash of key.
  @member key Key.
  @member value Value stored.