
theorem Th9:
  for S1,S2 being non empty ManySortedSign st S1 is not void or S2
  is not void holds S1+*S2 is non void
proof
  let S1,S2 be non empty ManySortedSign;

  assume
A1: S1 is not void or S2 is not void;
  the carrier' of S1+*S2 = (the carrier' of S1) \/ the carrier' of S2 by Def2;
  hence the carrier' of S1+*S2 is non empty by A1;
end;
