code: def _merge_event_and_control_columns(event_col, control_col):
      if control_col is None:
          return event_col
      else:
          return event_col.combine(
              control_col, _combine_event_and_control
          )
  
imports: import numpy
  import pandas
  from fairlearn.utils._input_validation import _MESSAGE_RATIO_NOT_IN_RANGE, _validate_and_reformat_input
  from .error_rate import ErrorRate
  from .moment import _ALL, _EVENT, _GROUP_ID, _LABEL, _SIGN, ClassificationMoment
label_desc: Use image node_id 2 for calling a global function with example usage: _merge_event_and_control_columns(event_col, control_col) and returns: event_col, event_col