theorem Th28:
  f tolerates g iff f c= f +* g
proof
  thus f tolerates g implies f c= (f +* g)
  proof
    dom f \/ dom g = dom(f +* g) by Def1;
    then
A1: dom f c= dom(f +* g) by XBOOLE_1:7;
    assume f tolerates g;
    then for x being object st x in dom f holds (f +* g).x = f.x by Th15;
    hence thesis by A1,GRFUNC_1:2;
  end;
  thus thesis by Th27,PARTFUN1:54;
end;
