Extension of Lipschitz mappings on metric trees

Let Y be a tree metric apace (i.e.satisfies the four-point condition, e.g., a graph tree with the graph metric).If X is a subset of Y and / is a mapping of X into a Banach space Z with Lipschitz constant L, then / can be extended on the whole Y with Lipschitz constant at most C • L t where C is an absolute constant.The extension depends linearly and continuously on /.