theorem
  for f,g,h being Function st dom g = dom h holds f +* g +* h = f +* h
proof
  let f,g,h be Function;
  assume
A1: dom g = dom h;
  thus f +* g +* h = f +* (g +* h) by Th14
    .= f +* h by A1,Th19;
end;
