(** Up to now, we've used the more manual part of Coq's tactic
    facilities.  In this chapter, we'll learn more about some of Coq's
    powerful automation features: proof search via the [auto] tactic,
    automated forward reasoning via the [Ltac] hypothesis matching
    machinery, and deferred instantiation of existential variables
    using [eapply] and [eauto].  Using these features together with
    Ltac's scripting facilities will enable us to make our proofs
    startlingly short!  Used properly, they can also make proofs more
    maintainable and robust to changes in underlying definitions.  A
    deeper treatment of [auto] and [eauto] can be found in the
    [UseAuto] chapter.

    There's another major category of automation we haven't discussed
    much yet, namely built-in decision procedures for specific kinds
    of problems: [omega] is one example, but there are others.  This
    topic will be deferred for a while longer.

    Our motivating example will be this proof, repeated with just a
    few small changes from the [Imp] chapter.  We will simplify
    this proof in several stages. *)

Ltac inv H := inversion H; subst; clear H.(* ################################################################# *)
(** * The [auto] Tactic *)

(** Thus far, our proof scripts mostly apply relevant hypotheses or
    lemmas by name, and one at a time. *)

Example auto_example_1 : forall (P Q R: Prop),
  (P -> Q) -> (Q -> R) -> P -> R.(** The [auto] tactic frees us from this drudgery by _searching_ for a
    sequence of applications that will prove the goal *)

Example auto_example_1' : forall (P Q R: Prop),
  (P -> Q) -> (Q -> R) -> P -> R.(** The [auto] tactic solves goals that are solvable by any combination of
     - [intros] and
     - [apply] (of hypotheses from the local context, by default). *)

(** Using [auto] is always "safe" in the sense that it will never fail
    and will never change the proof state: either it completely solves
    the current goal, or it does nothing. *)

(** Here is a more interesting example showing [auto]'s power: *)

Example auto_example_2 : forall P Q R S T U : Prop,
  (P -> Q) ->
  (P -> R) ->
  (T -> R) ->
  (S -> T -> U) ->
  ((P->Q) -> (P->S)) ->
  T ->
  P ->
  U.(** Proof search could, in principle, take an arbitrarily long time,
    so there are limits to how far [auto] will search by default. *)

Example auto_example_3 : forall (P Q R S T U: Prop),
  (P -> Q) ->
  (Q -> R) ->
  (R -> S) ->
  (S -> T) ->
  (T -> U) ->
  P ->
  U.(** When searching for potential proofs of the current goal,
    [auto] considers the hypotheses in the current context together
    with a _hint database_ of other lemmas and constructors.  Some
    common lemmas about equality and logical operators are installed
    in this hint database by default. *)

Example auto_example_4 : forall P Q R : Prop,
  Q ->
  (Q -> R) ->
  P \/ (Q /\ R).

(* Specifications: *)
Lemma le_antisym : forall n m: nat, (n <= m /\ m <= n) -> n = m.