reserve GS for GraphStruct;
reserve G,G1,G2,G3 for _Graph;
reserve e,x,x1,x2,y,y1,y2,E,V,X,Y for set;
reserve n,n1,n2 for Nat;
reserve v,v1,v2 for Vertex of G;

theorem
  _GraphSelectors c= dom G
proof
    let x be object;
    assume x in _GraphSelectors;
    then x = VertexSelector or x = EdgeSelector or x = SourceSelector or x =
    TargetSelector by ENUMSET1:def 2;
    hence x in dom G by Def10;
end;
