code: def _ul_per_mm(self, ul: float, func: str) -> float:
          
          sequence = self.ul_per_mm[func]
          return pipette_config.piecewise_volume_conversion(ul, sequence)
docs: :param ul: microliters as a float
          :param func: must be one of 'aspirate' or 'dispense'
          :return: microliters/mm as a float