Section Gram.
Context  {G : CFGV}.
Inductive  Pattern : (Terminal G) -> Type :=
| ptleaf : forall (T : Terminal G),
nat -> Pattern T
with Mixture : list (Terminal G) -> Type :=
| mtcons : forall {h: Terminal G}
{tl: list (Terminal G)},
Pattern h -> Mixture tl -> Mixture (h::tl).
Variable vc : VarSym G.
Fixpoint pBVars {gs}  (p : Pattern gs) : (list nat) :=
match p with
| ptleaf _ _ => []
end
with mBVars {lgs} (pts : Mixture lgs) : (list nat) :=
match pts with
| mtcons _ _ _ tl =>  mBVars  tl
end.

Lemma mBndngVarsAsNth :
forall mp (m : @Mixture mp),
mBVars m = [2].