prefix -- deference namespace prefix in node's context.
           Ascends parent nodes until found.

def getNamespace(self, prefix):
        """prefix -- deference namespace prefix in node's context.
           Ascends parent nodes until found.
        """
        namespace = None
        if prefix == 'xmlns':
            namespace = DOM.findDefaultNS(prefix, self.__node)
        else:
            try:
                namespace = DOM.findNamespaceURI(prefix, self.__node)
            except DOMException, ex:
                if prefix != 'xml':
                    raise SchemaError, '%s namespace not declared for %s'\
                        %(prefix, self.__node._get_tagName())
                namespace = XMLNS.XML
        return namespace