code: def concat(cls, *args):
          
          width = 0
          val = 0
          for arg in args:
              if not isinstance(arg, BinWord):
                  raise TypeError('need BinWord in concat')
              val |= arg._val << width
              width += arg._width
          return cls(width, val)
docs: Returns a BinWord made from concatenating several BinWords,
          in LSB-first order.