  We consider problems of access control for update of XML documents. In the
context of XML programming, types can be viewed as hedge automata, and static
type checking amounts to verify that a program always converts valid source
documents into also valid output documents. Given a set of update operations we
are particularly interested by checking safety properties such as preservation
of document types along any sequence of updates. We are also interested by the
related policy consistency problem, that is detecting whether a sequence of
authorized operations can simulate a forbidden one. We reduce these questions
to type checking problems, solved by computing variants of hedge automata
characterizing the set of ancestors and descendants of the initial document
type for the closure of parameterized rewrite rules.
