def parse_set_diff(source, info):
 items = [parse_set_imp_union(source, info)]
 while source.match('--'):
  items.append(parse_set_imp_union(source, info))
 if (len(items) == 1):
  return items[0]
 return SetDiff(info, items)

def parse_set_diff ( source , info ) : items = [ parse_set_imp_union ( source , info ) ] while source . match ( '--' ) : items . append ( parse_set_imp_union ( source , info ) ) if ( len ( items ) == 1 ) : return items [ 0 ] return SetDiff ( info , items )
parses a set difference .