# Library Coq.Init.Logic

Require Import Notations.

# Propositional connectives

True is the always true proposition
Inductive True : Prop :=
I : True.
False is the always false proposition
Inductive False : Prop :=.
not A, written ~A, is the negation of A
Definition not (A:Prop) := A -> False.
Notation "~ x" := (not x) : type_scope.
Hint Unfold not: core.
and A B, written A /\ B, is the conjunction of A and B
conj p q is a proof of A /\ B as soon as p is a proof of A and q a proof of B
proj1 and proj2 are first and second projections of a conjunction
Inductive and (A B:Prop) : Prop :=
conj : A -> B -> A /\ B
where "A /\ B" := (and A B) : type_scope.
Section Conjunction.
Variables A B : Prop.
Theorem proj1 : A /\ B -> A.
Theorem proj2 : A /\ B -> B.
End Conjunction.
or A B, written A \/ B, is the disjunction of A and B
Inductive or (A B:Prop) : Prop :=
| or_introl : A -> A \/ B
| or_intror : B -> A \/ B
where "A \/ B" := (or A B) : type_scope.
iff A B, written A <-> B, expresses the equivalence of A and B
Definition iff (A B:Prop) := (A -> B) /\ (B -> A).
Notation "A <-> B" := (iff A B) : type_scope.
Section Equivalence.
Theorem iff_refl : forall A:Prop, A <-> A.
Theorem iff_trans : forall A B C:Prop, (A <-> B) -> (B <-> C) -> (A <-> C).
Theorem iff_sym : forall A B:Prop, (A <-> B) -> (B <-> A).
End Equivalence.
Hint Unfold iff: extcore.
Some equivalences
Theorem neg_false : forall A : Prop, ~ A <-> (A <-> False).
Theorem and_cancel_l : forall A B C : Prop,
(B -> A) -> (C -> A) -> ((A /\ B <-> A /\ C) <-> (B <-> C)).
Theorem and_cancel_r : forall A B C : Prop,
(B -> A) -> (C -> A) -> ((B /\ A <-> C /\ A) <-> (B <-> C)).
Theorem and_comm : forall A B : Prop, A /\ B <-> B /\ A.
Theorem and_assoc : forall A B C : Prop, (A /\ B) /\ C <-> A /\ B /\ C.
Theorem or_cancel_l : forall A B C : Prop,
(B -> ~ A) -> (C -> ~ A) -> ((A \/ B <-> A \/ C) <-> (B <-> C)).
Theorem or_cancel_r : forall A B C : Prop,
(B -> ~ A) -> (C -> ~ A) -> ((B \/ A <-> C \/ A) <-> (B <-> C)).
Theorem or_comm : forall A B : Prop, (A \/ B) <-> (B \/ A).
Theorem or_assoc : forall A B C : Prop, (A \/ B) \/ C <-> A \/ B \/ C.
Backward direction of the equivalences above does not need assumptions
Theorem and_iff_compat_l : forall A B C : Prop,
(B <-> C) -> (A /\ B <-> A /\ C).
Theorem and_iff_compat_r : forall A B C : Prop,
(B <-> C) -> (B /\ A <-> C /\ A).
Theorem or_iff_compat_l : forall A B C : Prop,
(B <-> C) -> (A \/ B <-> A \/ C).
Theorem or_iff_compat_r : forall A B C : Prop,
(B <-> C) -> (B \/ A <-> C \/ A).
Lemma iff_and : forall A B : Prop, (A <-> B) -> (A -> B) /\ (B -> A).
Lemma iff_to_and : forall A B : Prop, (A <-> B) <-> (A -> B) /\ (B -> A).
(IF_then_else P Q R), written IF P then Q else R denotes either P and Q, or ~P and Q
Definition IF_then_else (P Q R:Prop) := P /\ Q \/ ~ P /\ R.
Notation "'IF' c1 'then' c2 'else' c3" := (IF_then_else c1 c2 c3)
(at level 200, right associativity) : type_scope.

# First-order quantifiers

ex P, or simply exists x, P x, or also exists x:A, P x, expresses the existence of an x of some type A in Set which satisfies the predicate P. This is existential quantification.
ex2 P Q, or simply exists2 x, P x & Q x, or also exists2 x:A, P x & Q x, expresses the existence of an x of type A which satisfies both predicates P and Q.
Universal quantification is primitively written forall x:A, Q. By symmetry with existential quantification, the construction all P is provided too.
Remark: exists x, Q denotes ex (fun x => Q) so that exists x, P x is in fact equivalent to ex (fun x => P x) which may be not convertible to ex P if P is not itself an abstraction
Inductive ex (A:Type) (P:A -> Prop) : Prop :=
ex_intro : forall x:A, P x -> ex (A:=A) P.
Inductive ex2 (A:Type) (P Q:A -> Prop) : Prop :=
ex_intro2 : forall x:A, P x -> Q x -> ex2 (A:=A) P Q.
Definition all (A:Type) (P:A -> Prop) := forall x:A, P x.
Notation "'exists' x , p" := (ex (fun x => p))
(at level 200, x ident, right associativity) : type_scope.
Notation "'exists' x : t , p" := (ex (fun x:t => p))
(at level 200, x ident, right associativity,
format "'[' 'exists' '/ ' x : t , '/ ' p ']'")
: type_scope.
Notation "'exists2' x , p & q" := (ex2 (fun x => p) (fun x => q))
(at level 200, x ident, p at level 200, right associativity) : type_scope.
Notation "'exists2' x : t , p & q" := (ex2 (fun x:t => p) (fun x:t => q))
(at level 200, x ident, t at level 200, p at level 200, right associativity,
format "'[' 'exists2' '/ ' x : t , '/ ' '[' p & '/' q ']' ']'")
: type_scope.
Derived rules for universal quantification
Section universal_quantification.
Variable A : Type.
Variable P : A -> Prop.
Theorem inst : forall x:A, all (fun x => P x) -> P x.
Theorem gen : forall (B:Prop) (f:forall y:A, B -> P y), B -> all P.
End universal_quantification.

# Equality

eq x y, or simply x=y expresses the equality of x and y. Both x and y must belong to the same type A. The definition is inductive and states the reflexivity of the equality. The others properties (symmetry, transitivity, replacement of equals by equals) are proved below. The type of x and y can be made explicit using the notation x = y :> A. This is Leibniz equality as it expresses that x and y are equal iff every property on A which is true of x is also true of y
Inductive eq (A:Type) (x:A) : A -> Prop :=
eq_refl : x = x :>A
where "x = y :> A" := (@eq A x y) : type_scope.
Notation "x = y" := (x = y :>_) : type_scope.
Notation "x <> y :> T" := (~ x = y :>T) : type_scope.
Notation "x <> y" := (x <> y :>_) : type_scope.
Implicit Arguments eq [ [A] ].
Implicit Arguments eq_ind [A].
Implicit Arguments eq_rec [A].
Implicit Arguments eq_rect [A].
Hint Resolve I conj or_introl or_intror eq_refl: core.
Hint Resolve ex_intro ex_intro2: core.
Section Logic_lemmas.
Theorem absurd : forall A C:Prop, A -> ~ A -> C.
Section equality.
Variables A B : Type.
Variable f : A -> B.
Variables x y z : A.
Theorem eq_sym : x = y -> y = x.
Opaque eq_sym.
Theorem eq_trans : x = y -> y = z -> x = z.
Opaque eq_trans.
Theorem f_equal : x = y -> f x = f y.
Opaque f_equal.
Theorem not_eq_sym : x <> y -> y <> x.
End equality.
Definition eq_ind_r :
forall (A:Type) (x:A) (P:A -> Prop), P x -> forall y:A, y = x -> P y.
Defined.
Definition eq_rec_r :
forall (A:Type) (x:A) (P:A -> Set), P x -> forall y:A, y = x -> P y.
Defined.
Definition eq_rect_r :
forall (A:Type) (x:A) (P:A -> Type), P x -> forall y:A, y = x -> P y.
Defined.
End Logic_lemmas.
Theorem f_equal2 :
forall (A1 A2 B:Type) (f:A1 -> A2 -> B) (x1 y1:A1)
(x2 y2:A2), x1 = y1 -> x2 = y2 -> f x1 x2 = f y1 y2.
Theorem f_equal3 :
forall (A1 A2 A3 B:Type) (f:A1 -> A2 -> A3 -> B) (x1 y1:A1)
(x2 y2:A2) (x3 y3:A3),
x1 = y1 -> x2 = y2 -> x3 = y3 -> f x1 x2 x3 = f y1 y2 y3.
Theorem f_equal4 :
forall (A1 A2 A3 A4 B:Type) (f:A1 -> A2 -> A3 -> A4 -> B)
(x1 y1:A1) (x2 y2:A2) (x3 y3:A3) (x4 y4:A4),
x1 = y1 -> x2 = y2 -> x3 = y3 -> x4 = y4 -> f x1 x2 x3 x4 = f y1 y2 y3 y4.
Theorem f_equal5 :
forall (A1 A2 A3 A4 A5 B:Type) (f:A1 -> A2 -> A3 -> A4 -> A5 -> B)
(x1 y1:A1) (x2 y2:A2) (x3 y3:A3) (x4 y4:A4) (x5 y5:A5),
x1 = y1 ->
x2 = y2 ->
x3 = y3 -> x4 = y4 -> x5 = y5 -> f x1 x2 x3 x4 x5 = f y1 y2 y3 y4 y5.
Notation sym_eq := eq_sym (only parsing).
Notation trans_eq := eq_trans (only parsing).
Notation sym_not_eq := not_eq_sym (only parsing).
Notation refl_equal := eq_refl (only parsing).
Notation sym_equal := eq_sym (only parsing).
Notation trans_equal := eq_trans (only parsing).
Notation sym_not_equal := not_eq_sym (only parsing).
Hint Immediate eq_sym not_eq_sym: core.
Basic definitions about relations and properties
Definition subrelation (A B : Type) (R R' : A->B->Prop) :=
forall x y, R x y -> R' x y.
Definition unique (A : Type) (P : A->Prop) (x:A) :=
P x /\ forall (x':A), P x' -> x=x'.
Definition uniqueness (A:Type) (P:A->Prop) := forall x y, P x -> P y -> x = y.
Unique existence
Notation "'exists' ! x , P" := (ex (unique (fun x => P)))
(at level 200, x ident, right associativity,
format "'[' 'exists' ! '/ ' x , '/ ' P ']'") : type_scope.
Notation "'exists' ! x : A , P" :=
(ex (unique (fun x:A => P)))
(at level 200, x ident, right associativity,
format "'[' 'exists' ! '/ ' x : A , '/ ' P ']'") : type_scope.
Lemma unique_existence : forall (A:Type) (P:A->Prop),
((exists x, P x) /\ uniqueness P) <-> (exists! x, P x).

# Being inhabited

The predicate inhabited can be used in different contexts. If A is thought as a type, inhabited A states that A is inhabited. If A is thought as a computationally relevant proposition, then inhabited A weakens A so as to hide its computational meaning. The so-weakened proof remains computationally relevant but only in a propositional context.
Inductive inhabited (A:Type) : Prop := inhabits : A -> inhabited A.
Hint Resolve inhabits: core.
Lemma exists_inhabited : forall (A:Type) (P:A->Prop),
(exists x, P x) -> inhabited A.
Declaration of stepl and stepr for eq and iff
Lemma eq_stepl : forall (A : Type) (x y z : A), x = y -> x = z -> z = y.
Declare Left Step eq_stepl.
Declare Right Step eq_trans.
Lemma iff_stepl : forall A B C : Prop, (A <-> B) -> (A <-> C) -> (C <-> B).
Declare Left Step iff_stepl.
Declare Right Step iff_trans.