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
  W is trivial iff W.reverse() is trivial
proof
  thus W is trivial implies W.reverse() is trivial;
  assume W.reverse() is trivial;
  then len W.reverse() = 1 by Lm55;
  then len W = 1 by FINSEQ_5:def 3;
  hence thesis by Lm55;
end;
