When type checking applications using lib2to3, the difference in types for lib2to3.pytree.Base.children versus Node.children makes it difficult to accept both leaves and nodes and programatically work with child nodes/leaves.  Base/Leaf both have `children` defined as an empty Tuple, while Node defines it as an empty list.  It would be more useful for Base/Leaf to also use an empty list, so that the type is the same, regardless of which type of object you are given.