reserve x,y for set;
reserve N for PT_net_Str;

theorem Th11:
  for N being Pnet for x being Element of Elements(N) holds Post(N
  ,x) c= Elements(N)
proof
  let N be Pnet;
  let x be Element of Elements(N);
  for y being object holds y in Post(N,x) implies y in Elements(N) by Def7;
  hence thesis by TARSKI:def 3;
end;
