code: def width_tuple(value):
      
      if value is None:
          res = 'fit', 0, 0
      elif not isinstance(value, (list, tuple)):
          raise VdtTypeError(value)
      elif value[0] not in ['fit', 'weight']:
          raise VdtTypeError(value)
      if value[0] == 'fit':
          if not isinstance(value[1], int) or not isinstance(value[2], int):
              VdtTypeError(value)
          res = 'fit', int(value[1]), int(value[2])
      else:
          if not isinstance(value[1], int):
              VdtTypeError(value)
          res = 'weight', int(value[1])
      return res
docs: test if value is a valid width indicator (for a sub-widget in a column).
      This can either be
      ('fit', min, max): use the length actually needed for the content, padded
                         to use at least width min, and cut of at width max.
                         Here, min and max are positive integers or 0 to disable
                         the boundary.
      ('weight',n): have it relative weight of n compared to other columns.
                    Here, n is an int.