pub fn leaf<Ns, Tag, Leaf, Att, Val>( leaf: Leaf ) -> Node<Ns, Tag, Leaf, Att, Val>where Ns: PartialEq + Clone + Debug, Tag: PartialEq + Debug, Leaf: PartialEq + Clone + Debug, Att: PartialEq + Clone + Debug, Val: PartialEq + Clone + Debug,
create a leaf node