theorem
  for f, g, h being Function st f c= g & dom f misses dom h holds f c= g +* h
proof
  let f, g, h be Function;
  assume f c= g;
  then
A1: f +* h c= g +* h by Th123;
  assume dom f misses dom h;
  then f c= f +* h by Th32;
  hence thesis by A1;
end;
