(** This defines a new tactical called [simpl_and_try] that takes one
    tactic [c] as an argument and is defined to be equivalent to the
    tactic [simpl; try c].  Now writing "[simpl_and_try reflexivity.]"
    in a proof will be the same as writing "[simpl; try
    reflexivity.]" *)

(* ================================================================= *)
(** ** The [omega] Tactic *)

(** The [omega] tactic implements a decision procedure for a subset of
    first-order logic called _Presburger arithmetic_.  It is based on
    the Omega algorithm invented in 1991 by William Pugh [Pugh 1991].

    If the goal is a universally quantified formula made out of

      - numeric constants, addition ([+] and [S]), subtraction ([-]
        and [pred]), and multiplication by constants (this is what
        makes it Presburger arithmetic),

      - equality ([=] and [<>]) and inequality ([<=]), and

      - the logical connectives [/\], [\/], [~], and [->],

    then invoking [omega] will either solve the goal or tell you that
    it is actually false. *)

Example silly_presburger_example : forall m n o p,
  m + n <= n + o /\ o + 3 = p + 3 ->
  m <= p.Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum  : forall (n: nat),
      aevalR (ANum n) n
  | E_APlus : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (APlus e1 e2) (n1 + n2)
  | E_AMinus: forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMinus e1 e2) (n1 - n2)
  | E_AMult : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMult e1 e2) (n1 * n2).Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum  : forall (n: nat),
      aevalR (ANum n) n
  | E_APlus : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (APlus e1 e2) (n1 + n2)
  | E_AMinus: forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMinus e1 e2) (n1 - n2)
  | E_AMult : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMult e1 e2) (n1 * n2).Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum  : forall (n: nat),
      aevalR (ANum n) n
  | E_APlus : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (APlus e1 e2) (n1 + n2)
  | E_AMinus: forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMinus e1 e2) (n1 - n2)
  | E_AMult : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMult e1 e2) (n1 * n2).Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum  : forall (n: nat),
      aevalR (ANum n) n
  | E_APlus : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (APlus e1 e2) (n1 + n2)
  | E_AMinus: forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMinus e1 e2) (n1 - n2)
  | E_AMult : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMult e1 e2) (n1 * n2).Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum  : forall (n: nat),
      aevalR (ANum n) n
  | E_APlus : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (APlus e1 e2) (n1 + n2)
  | E_AMinus: forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMinus e1 e2) (n1 - n2)
  | E_AMult : forall (e1 e2: aexp) (n1 n2: nat),
      aevalR e1 n1 ->
      aevalR e2 n2 ->
      aevalR (AMult e1 e2) (n1 * n2).(** It will be convenient to have an infix notation for
    [aevalR].  We'll write [e \\ n] to mean that arithmetic expression
    [e] evaluates to value [n].  (This notation is one place where the
    limitation to ASCII symbols becomes a little bothersome.  The
    standard notation for the evaluation relation is a double
    down-arrow.  We'll typeset it like this in the HTML version of the
    notes and use a double slash as the closest approximation in [.v]
    files.)  *)

Notation "e '\\' n"
         := (aevalR e n)
            (at level 50, left associativity)
         : type_scope.Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum : forall (n:nat),
      (ANum n) \\ n
  | E_APlus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (APlus e1 e2) \\ (n1 + n2)
  | E_AMinus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMinus e1 e2) \\ (n1 - n2)
  | E_AMult :  forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMult e1 e2) \\ (n1 * n2)

  where "e '\\' n" := (aevalR e n) : type_scope.Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum : forall (n:nat),
      (ANum n) \\ n
  | E_APlus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (APlus e1 e2) \\ (n1 + n2)
  | E_AMinus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMinus e1 e2) \\ (n1 - n2)
  | E_AMult :  forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMult e1 e2) \\ (n1 * n2)

  where "e '\\' n" := (aevalR e n) : type_scope.Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum : forall (n:nat),
      (ANum n) \\ n
  | E_APlus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (APlus e1 e2) \\ (n1 + n2)
  | E_AMinus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMinus e1 e2) \\ (n1 - n2)
  | E_AMult :  forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMult e1 e2) \\ (n1 * n2)

  where "e '\\' n" := (aevalR e n) : type_scope.Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum : forall (n:nat),
      (ANum n) \\ n
  | E_APlus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (APlus e1 e2) \\ (n1 + n2)
  | E_AMinus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMinus e1 e2) \\ (n1 - n2)
  | E_AMult :  forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMult e1 e2) \\ (n1 * n2)

  where "e '\\' n" := (aevalR e n) : type_scope.Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum : forall (n:nat),
      (ANum n) \\ n
  | E_APlus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (APlus e1 e2) \\ (n1 + n2)
  | E_AMinus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMinus e1 e2) \\ (n1 - n2)
  | E_AMult :  forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMult e1 e2) \\ (n1 * n2)

  where "e '\\' n" := (aevalR e n) : type_scope.Inductive aevalR : aexp -> nat -> Prop :=
  | E_ANum : forall (n:nat),
      (ANum n) \\ n
  | E_APlus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (APlus e1 e2) \\ (n1 + n2)
  | E_AMinus : forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMinus e1 e2) \\ (n1 - n2)
  | E_AMult :  forall (e1 e2: aexp) (n1 n2 : nat),
      (e1 \\ n1) -> (e2 \\ n2) -> (AMult e1 e2) \\ (n1 * n2)

  where "e '\\' n" := (aevalR e n) : type_scope.

Theorem aeval_iff_aevalR : forall a n, (a \\ n) <-> aeval a = n.