theorem Th103:
 for x,y being object holds
  not x in rng f implies f+~(x,y) = f
proof let x,y be object;
  assume not x in rng f;
  then dom (x .--> y) misses rng f by ZFMISC_1:50;
  then (x .--> y)*f = {} by RELAT_1:44;
  hence thesis;
end;
