code: def is_simple_unit(self, unit):
          
          if self.unit_denom or len(self.unit_numer) > 1:
              return False
  
          if not self.unit_numer:
              # Empty string historically means no unit
              return unit == ''
  
          return self.unit_numer[0] == unit
docs: Return True iff the unit is simple (as above) and matches the given
          unit.