
theorem
  for G1, G2 being non-multi _Graph, f being PVertexMapping of G1, G2
  for v being Vertex of G1 st f is isomorphism
  holds v.degree() = (f/.v).degree()
proof
  let G1, G2 be non-multi _Graph, f be PVertexMapping of G1, G2;
  let v be Vertex of G1;
  assume f is isomorphism;
  hence v.degree() = ((PVM2PGM f)_V/.v).degree() by Th99, GLIB_011:47
    .= (f/.v).degree();
end;
