From Undecidability.L Require Export Tactics.LTactics Functions.Encoding Datatypes.LBool L.
(* * Definition of L-decidability *)
Definition decides (u:term) P := forall (s:term), exists b : bool, (app u (ext s) == ext b /\ (if b then P s else ~P s)).
Definition ldec (P : term -> Prop) :=
exists u : term, proc u /\ decides u P.
(* * Complement, conj and disj of predicates *)
Definition complement (P : term -> Prop) := fun t => ~ P t.
Definition conj (P : term -> Prop) (Q : term -> Prop) := fun t => P t /\ Q t.
Definition disj (P : term -> Prop) (Q : term -> Prop) := fun t => P t \/ Q t.
(* * Deciders for complement, conj and disj of ldec predicates *)
Definition tcompl (u : term) : term := Eval cbn in λ x, !!(ext negb) (!!u x).
Definition tconj (u v : term) : term := Eval cbn in λ x, !!(ext andb) (!!u x) (!!v x).
Definition tdisj (u v : term) : term := Eval cbn in λ x, !!(ext orb) (!!u x) (!!v x).
Hint Unfold tcompl tconj tdisj : Lrewrite.
Hint Unfold tcompl tconj tdisj : LProc.
(* * L-decidable predicates are closed under complement, conj and disj *)

Lemma ldec_complement P : ldec P -> ldec (complement P).