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

theorem Th13:
  for oc being directed non empty Chain of G holds (vertex-seq oc)
  .(len oc +1) = (the Target of G).(oc.len oc)
proof
  let oc be directed non empty Chain of G;
  1 in dom oc by FINSEQ_5:6;
  then 1<=len oc by FINSEQ_3:25;
  hence thesis by Th10;
end;
