
theorem Th20:
  for X,Y being set, f being Function holds (X \/ Y)-indexing f =
  (X-indexing f) \/ (Y-indexing f)
proof
  let X,Y be set, f be Function;
A1: X-indexing f tolerates Y-indexing f by Th19;
  (X \/ Y)-indexing f = (X-indexing f) +* (Y-indexing f) by Th18;
  hence thesis by A1,FUNCT_4:30;
end;
