reserve G for _Graph;
reserve G2 for _Graph, G1 for Supergraph of G2;
reserve V for set;

theorem Th90:
  for G2, V for G1 being addVertices of G2, V, v being object
  st v in V holds v is Vertex of G1
proof
  let G2, V;
  let G1 be addVertices of G2, V;
  let v be object;
  assume v in V;
  then v in V \/ the_Vertices_of G2 by XBOOLE_0:def 3;
  hence thesis by Def10;
end;
