code: def parse_units(self, stream):
          
          value = b''
          self.expect(stream, self.begin_units)
  
          while not self.has_next(self.end_units, stream):
              if self.has_eof(stream):
                  self.raise_unexpected_eof(stream)
              value += stream.read(1)
  
          self.expect(stream, self.end_units)
          return value.strip(b''.join(self.whitespace)).decode('utf-8')
docs: UnitsExpression ::=
              UnitsStart WhiteSpace* UnitsValue WhiteSpace* UnitsEnd