reserve G,G1,G2 for _Graph;
reserve W,W1,W2 for Walk of G;
reserve e,x,y,z for set;
reserve v for Vertex of G;
reserve n,m for Element of NAT;

theorem
  x in W.vertices() iff ex n being odd Element of NAT st n <= len W & W.
  n = x by Lm45;
