
theorem
  for G1 being _Graph, G2 being G1-isomorphic _Graph
  holds G1 is G2-isomorphic
proof
  let G1 be _Graph, G2 be G1-isomorphic _Graph;
  consider F being PGraphMapping of G1, G2 such that
    A1: F is isomorphism by Def23;
  reconsider F as one-to-one PGraphMapping of G1, G2 by A1;
  take F";
  thus thesis by A1, Th75;
end;
