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
  n in dom W.reverse() implies W.reverse().n = W.(len W - n + 1) &
  (len W - n + 1) in dom W by Lm8;
