let G be _Graph; :: thesis: _GraphSelectors c= dom G
let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in _GraphSelectors or x in dom G )
assume x in _GraphSelectors ; :: thesis: x in dom G
then ( x = VertexSelector or x = EdgeSelector or x = SourceSelector or x = TargetSelector ) by ENUMSET1:def 2;
hence x in dom G by Def10; :: thesis: verum