theorem Th87:
  for f,g,h being Function st f c= h & g c= h holds f +* g c= h
proof
  let f,g,h be Function;
  assume f c= h & g c= h;
  then
A1: f \/ g c= h by XBOOLE_1:8;
  f +* g c= f \/ g by Th29;
  hence thesis by A1;
end;
