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 W1.append(W2) implies n in dom W1 or ex k being Element of
  NAT st k < len W2 & n = len W1 + k by Lm14;
