Definition ctx_S :=
    F3 :: rev F2 ++ rev F1.

(* Specifications: *)
Lemma drv_prv s u v : derivable R u v -> (@prv s b ctx_S (Pr (enc u) (enc v))).
Lemma BPCP_prv' s : BPCP' R -> @prv s b nil F.
Lemma form_discrete {b : logic} : discrete (form b).