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 Th21:
  W.first() = W.reverse().last() & W.last() = W.reverse().first()
proof
  len W = len W.reverse() by FINSEQ_5:def 3;
  hence W.first() = W.reverse().last() by FINSEQ_5:62;
  thus thesis by FINSEQ_5:62;
end;
