theorem Th25:
  g c= f +* g
proof
  dom(f +* g) = dom f \/ dom g by Def1;
  then
A1: dom g c= dom(f +* g) by XBOOLE_1:7;
  for x being object st x in dom g holds (f+*g).x = g.x by Th13;
  hence thesis by A1,GRFUNC_1:2;
end;
