theorem Th65:
  1 <= len W.vertexSeq()
proof
  now
    assume len W.vertexSeq() < 1;
    then len W.vertexSeq() < 0 + 1;
    then len W.vertexSeq() = 0 by NAT_1:13;
    then len W + 1 = 2 * 0 by Def14;
    hence contradiction;
  end;
  hence thesis;
end;
