code: def convert_units_to_base_units(units):
      
      total_factor = 1
      new_units = []
      for unit in units:
          if unit not in BASE_UNIT_CONVERSIONS:
              continue
  
          factor, new_unit = BASE_UNIT_CONVERSIONS[unit]
          total_factor *= factor
          new_units.append(new_unit)
  
      new_units.sort()
      return total_factor, tuple(new_units)
docs: Convert a set of units into a set of "base" units.
  
      Returns a 2-tuple of `factor, new_units`.