code: def _check_ts_freq(ts_freq, DUMMIES):
      if (ts_freq is not None) & (
          ts_freq not in DUMMIES["ts_frequency"].unique()
      ):
          raise ValueError(
              "Invalid ts_freq specified, must be in: "
              + ", ".join(DUMMIES["ts_frequency"].unique())
          )
  
file_path: sktime/sktime/transformations/series/date.py
imports: import warnings
  import numpy
  import pandas
  from sktime.transformations.base import BaseTransformer
label_desc: Use image node_id 3 for calling a global function with example usage: _check_ts_freq(ts_freq, DUMMIES) without return types