reserve G for Graph,
  k, m, n for Nat;
reserve G for non void Graph;

theorem Th9:
  for oc being non empty directed Chain of G holds len vertex-seq
  oc = len oc +1
proof
  let oc be non empty directed Chain of G;
  vertex-seq oc is_vertex_seq_of oc by GRAPH_2:def 10;
  hence thesis;
end;
