langchain/chains/query_constructor/parser.py/QueryTransformer/_match_func_name
class QueryTransformer:    def _match_func_name(self, func_name: str) -> Union[Operator, Comparator]:
        if func_name in set(Comparator):
            if self.allowed_comparators is not None:
                if func_name not in self.allowed_comparators:
                    raise ValueError(
                        f"Received disallowed comparator {func_name}. Allowed "
                        f"comparators are {self.allowed_comparators}"
                    )
            return Comparator(func_name)
        elif func_name in set(Operator):
            if self.allowed_operators is not None:
                if func_name not in self.allowed_operators:
                    raise ValueError(
                        f"Received disallowed operator {func_name}. Allowed operators"
                        f" are {self.allowed_operators}"
                    )
            return Operator(func_name)
        else:
            raise ValueError(
                f"Received unrecognized function {func_name}. Valid functions are "
                f"{list(Operator) + list(Comparator)}"
            )