End CFGs.
Definition CFP' (G : cfg) := exists x, L G x /\ x = List.rev x.
Definition CFI' '(G1, G2) :=
exists x, L G1 x /\ L G2 x.
Section Post_CFG.
Variable R : SRS.
Variable a : symbol.
Definition Sigma := sym R ++ [a].
Definition S : symbol := fresh Sigma.
Definition G := (S, (S,[S]) :: map (fun '(u / v) => (S, u ++ [S] ++ v)) R ++ map (fun '(u / v) => (S, u ++ [a] ++ v)) R).

Lemma Post_CFG_1' A :
A <<= R -> A = [] \/ rewt G [S] (sigma a A).