code: def _check_operator(op: str):
      """
      Check that operator is one of
      `>`, `>=`, `==`, `!=`, `<`, `<=`.
  
      Used in `conditional_join`.
      """
      sequence_of_operators = {op.value for op in _JoinOperator}
      if op not in sequence_of_operators:
          raise ValueError(
              "The conditional join operator "
              f"should be one of {sequence_of_operators}"
          )
  
imports: from __future__ import annotations
  import operator
  import warnings
  from typing import Any, Hashable, Literal, Optional, Union
  import numpy
  import pandas
  import pandas_flavor
  from pandas.api.types import is_datetime64_dtype, is_dtype_equal, is_extension_array_dtype, is_numeric_dtype, is_timedelta64_dtype
  from pandas.core.reshape.merge import _MergeOperation
  from janitor.functions.utils import _generic_func_cond_join, _JoinOperator, _keep_output, col, greater_than_join_types, less_than_join_types
  from janitor.utils import check, check_column, find_stack_level
label_desc: Use image node_id 2 for calling a global function with example usage: _check_operator(op) without return types