theorem Th4:
  P c= P1 & Q c= Q1 implies P + Q c= P1 + Q1
proof
  assume
A1: P c= P1 & Q c= Q1;
  let x be object;
  assume x in P + Q;
  then ex g, t being Element of H st x = g + t & g in P & t in Q;
  hence thesis by A1;
end;
