theorem Th2:
  for f,g being Function st X c= dom(g*f) holds f.:X c= dom g
proof
  let f,g be Function such that
A1: X c= dom(g*f);
  let y be object;
  assume y in f.:X;
  then ex x being object st x in dom f & x in X & y = f.x by FUNCT_1:def 6;
  hence thesis by A1,FUNCT_1:11;
end;
