reserve x,y for object,
  f for Function;

theorem Th38:
  [x,y] in *graph f iff x in dom f & y in f.x
proof
A1: [x,y] in *graph f iff [y,x] in Union disjoin f by RELAT_1:def 7;
  [y,x]`1 = y & [y,x]`2 = x;
  hence thesis by A1,CARD_3:22;
end;
