We introduce a method for building a minimal XML type
(belonging to standard class of regular tree grammars) as an extension of
other given types. Not only do we propose an easy-to-handle XML type
evolution method, but we prove that this method computes the smallest
extension of a given tree grammar, respecting pre-established constraints.
We also adapt our technique to an interactive context, where an advised
user is guided to build a new XML type from existing ones. A basic
prototype of our tool is implemented.