for resolving import statements in Schema instance
           schema -- schema instance
           _imported_schemas

def addImportSchema(self, schema):
        """for resolving import statements in Schema instance
           schema -- schema instance
           _imported_schemas 
        """
        if not isinstance(schema, XMLSchema):
            raise TypeError, 'expecting a Schema instance'
        if schema.targetNamespace != self.targetNamespace:
            self._imported_schemas[schema.targetNamespace] = schema
        else:
            raise SchemaError, 'import schema bad targetNamespace'