docs: 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'