reserve G for Graph,
  v, v1, v2 for Vertex of G,
  c for Chain of G,
  p, p1, p2 for Path of G,
  vs, vs1, vs2 for FinSequence of the carrier of G,
  e, X for set,
  n, m for Nat;

theorem Th16:
  G-VSet {} = {}
proof
  assume not thesis;
  then consider x being object such that
A1: x in G-VSet {} by XBOOLE_0:def 1;
  ex v being Vertex of G st x = v & ex e being Element of the carrier' of G
  st e in {} & (v = (the Source of G).e or v = (the Target of G ).e) by A1;
  hence contradiction;
end;
