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
 for x,y being object holds
  W is_Walk_from x,y iff W.1 = x & W.(len W) = y;
