code: def type(value):
          
  
          valid_transforms = frozenset([
              'array', 'copy', 'cross', 'facet', 'filter',
              'flatten', 'fold', 'formula', 'slice', 'sort', 'stats',
              'truncate', 'unique', 'window', 'zip', 'force', 'geo', 'geopath',
              'link', 'pie', 'stack', 'treemap', 'wordcloud'
          ])
  
          if value not in valid_transforms:
              raise ValueError('Transform type must be'
                               ' one of {0}'.format(str(valid_transforms)))
docs: string: property name in which to store the computed transform
          value.
  
          The valid transform types are as follows:
          'array', 'copy', 'cross', 'facet', 'filter', 'flatten', 'fold',
          'formula', 'slice', 'sort', 'stats', 'truncate', 'unique', 'window',
          'zip', 'force', 'geo', 'geopath', 'link', 'pie', 'stack', 'treemap',
          'wordcloud'