Section INVARIANT6_bis.
(* these two lemmas are just negations of the previous ones *)
(* the parent of a node cannot be the which receives a gp for the first time *)
(* the ancestor of a node cannot be the which receives a gp for the first time *)

Lemma ancestor_has_positive_st :
forall c : Config,
legal c -> forall s1 s2 : Site, ancestor c s1 s2 -> (st c s1 > 0)%Z.