code: def ts_func(ts, x):
      return x - ts.month
  
file_path: darts/darts/tests/dataprocessing/transformers/test_mappers.py
imports: import numpy
  import pandas
  from darts import TimeSeries
  from darts.dataprocessing.transformers.mappers import InvertibleMapper, Mapper
  from darts.utils.timeseries_generation import constant_timeseries, linear_timeseries
label_desc: Use image node_id 3 for calling the TestMappers obj's underlying member method code with example usage: obj.ts_func(ts, x) and returns: unknown