for resolving include statements in Schema instance
           schemaLocation -- schema location
           schema -- schema instance
           _included_schemas

def addIncludeSchema(self, schemaLocation, schema):
        """for resolving include statements in Schema instance
           schemaLocation -- schema location
           schema -- schema instance
           _included_schemas 
        """
        if not isinstance(schema, XMLSchema):
            raise TypeError, 'expecting a Schema instance'
        if not schema.targetNamespace or\
             schema.targetNamespace == self.targetNamespace:
            self._included_schemas[schemaLocation] = schema
        else:
            raise SchemaError, 'include schema bad targetNamespace'