code: def _combine_event_and_control(event: str, control: str) -> str:
      if pd.notnull(control):
          return _CTRL_EVENT_FORMAT.format(control, event)
      else:
          return event
  
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 1 for calling a global function with example usage: _combine_event_and_control(event, control) and returns: _CTRL_EVENT_FORMAT, event