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
  m is odd & m <= n implies W.cut(1,n).cut(1,m) = W.cut(1,m) by Lm20;
