code: def base_fields(self):
          '''return number of non-extended fields'''
          if self.extensions_start is None:
              return len(self.fields)
          return len(self.fields[:self.extensions_start]f base_fields(self):
          '''return number of non-extended fields'''
          if self.extensions_start is None:
              return len(self.fields)
          return len(self.fields[:self.extensions_start])
docs: return number of non-extended fields