invariant { not L.mem !t !stackNodes } (* this will be usefull to prove that !t is in graph *)
(*invariant { not L.mem !t !stackNodes }*) (* this will be usefull to prove that !t is in graph after the push ---> but this is false! think of the case when the graph is cyclic *)
(* I4d from Hubert and Marché's paper and something related to line 63-65 from Leino's *)
invariant { forall n : loc. S.mem n graph /\ n <> null /\ not L.mem n !stackNodes ->