reserve X,Y for set, x,y,z for object, i,j,n for natural number;

theorem Th3:
  for f,g being Function holds f tolerates g+*f
  proof
    let f,g be Function;
    let x; assume x in dom f /\ dom (g+*f);
    then x in dom f /\ (dom g \/ dom f) = dom f by FUNCT_4:def 1,XBOOLE_1:21;
    hence (g+*f).x = f.x by FUNCT_4:13;
  end;
