code: def addImportSchema(self, schema):
          
          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'
docs: for resolving import statements in Schema instance
             schema -- schema instance
             _imported_schemas