This module provides a state filter as a main function for validating XML documents.
The real validation is done with pure filters from module
Author : .\artin Schmidt
monadic filter for validating and transforming a wellformed document.
the main function for validation.
the input tree must consist of a root node with a complete document and DTD. Result is the single element list containing same tree but tranformed with respect to the DTD, or, in case of errors, the root with an empty list of children