theorem Th60:
  for x be object st x in dom(f+g+h) holds (f+g+h).x=f.x+g.x+h.x
proof
  let x be object;
  assume
A1: x in dom(f+g+h);
  dom(f+g+h) = dom(f+g) /\ dom h by VALUED_1:def 1;
  then x in dom(f+g) by A1,XBOOLE_0:def 4;
  then f.x + g.x + h.x = (f+g).x + h.x by VALUED_1:def 1;
  hence thesis by A1,VALUED_1:def 1;
end;
