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diff --git a/doc/sphinx/proofs/automatic-tactics/logic.rst b/doc/sphinx/proofs/automatic-tactics/logic.rst new file mode 100644 index 0000000000..acf64ae437 --- /dev/null +++ b/doc/sphinx/proofs/automatic-tactics/logic.rst @@ -0,0 +1,294 @@ +.. _decisionprocedures: + +============================== +Solvers for logic and equality +============================== + +.. tacn:: tauto + :name: tauto + + This tactic implements a decision procedure for intuitionistic propositional + calculus based on the contraction-free sequent calculi LJT* of Roy Dyckhoff + :cite:`Dyc92`. Note that :tacn:`tauto` succeeds on any instance of an + intuitionistic tautological proposition. :tacn:`tauto` unfolds negations and + logical equivalence but does not unfold any other definition. + +.. example:: + + The following goal can be proved by :tacn:`tauto` whereas :tacn:`auto` would + fail: + + .. coqtop:: reset all + + Goal forall (x:nat) (P:nat -> Prop), x = 0 \/ P x -> x <> 0 -> P x. + intros. + tauto. + +Moreover, if it has nothing else to do, :tacn:`tauto` performs introductions. +Therefore, the use of :tacn:`intros` in the previous proof is unnecessary. +:tacn:`tauto` can for instance for: + +.. example:: + + .. coqtop:: reset all + + Goal forall (A:Prop) (P:nat -> Prop), A \/ (forall x:nat, ~ A -> P x) -> forall x:nat, ~ A -> P x. + tauto. + +.. note:: + In contrast, :tacn:`tauto` cannot solve the following goal + :g:`Goal forall (A:Prop) (P:nat -> Prop), A \/ (forall x:nat, ~ A -> P x) ->` + :g:`forall x:nat, ~ ~ (A \/ P x).` + because :g:`(forall x:nat, ~ A -> P x)` cannot be treated as atomic and + an instantiation of `x` is necessary. + +.. tacv:: dtauto + :name: dtauto + + While :tacn:`tauto` recognizes inductively defined connectives isomorphic to + the standard connectives ``and``, ``prod``, ``or``, ``sum``, ``False``, + ``Empty_set``, ``unit``, ``True``, :tacn:`dtauto` also recognizes all inductive + types with one constructor and no indices, i.e. record-style connectives. + +.. tacn:: intuition @tactic + :name: intuition + + The tactic :tacn:`intuition` takes advantage of the search-tree built by the + decision procedure involved in the tactic :tacn:`tauto`. It uses this + information to generate a set of subgoals equivalent to the original one (but + simpler than it) and applies the tactic :n:`@tactic` to them :cite:`Mun94`. If + this tactic fails on some goals then :tacn:`intuition` fails. In fact, + :tacn:`tauto` is simply :g:`intuition fail`. + + .. example:: + + For instance, the tactic :g:`intuition auto` applied to the goal:: + + (forall (x:nat), P x) /\ B -> (forall (y:nat), P y) /\ P O \/ B /\ P O + + internally replaces it by the equivalent one:: + + (forall (x:nat), P x), B |- P O + + and then uses :tacn:`auto` which completes the proof. + +Originally due to César Muñoz, these tactics (:tacn:`tauto` and +:tacn:`intuition`) have been completely re-engineered by David Delahaye using +mainly the tactic language (see :ref:`ltac`). The code is +now much shorter and a significant increase in performance has been noticed. +The general behavior with respect to dependent types, unfolding and +introductions has slightly changed to get clearer semantics. This may lead to +some incompatibilities. + +.. tacv:: intuition + + Is equivalent to :g:`intuition auto with *`. + +.. tacv:: dintuition + :name: dintuition + + While :tacn:`intuition` recognizes inductively defined connectives + isomorphic to the standard connectives ``and``, ``prod``, ``or``, ``sum``, ``False``, + ``Empty_set``, ``unit``, ``True``, :tacn:`dintuition` also recognizes all inductive + types with one constructor and no indices, i.e. record-style connectives. + +.. flag:: Intuition Negation Unfolding + + Controls whether :tacn:`intuition` unfolds inner negations which do not need + to be unfolded. This flag is on by default. + +.. tacn:: rtauto + :name: rtauto + + The :tacn:`rtauto` tactic solves propositional tautologies similarly to what + :tacn:`tauto` does. The main difference is that the proof term is built using a + reflection scheme applied to a sequent calculus proof of the goal. The search + procedure is also implemented using a different technique. + + Users should be aware that this difference may result in faster proof-search + but slower proof-checking, and :tacn:`rtauto` might not solve goals that + :tacn:`tauto` would be able to solve (e.g. goals involving universal + quantifiers). + + Note that this tactic is only available after a ``Require Import Rtauto``. + +.. tacn:: firstorder + :name: firstorder + + The tactic :tacn:`firstorder` is an experimental extension of :tacn:`tauto` to + first- order reasoning, written by Pierre Corbineau. It is not restricted to + usual logical connectives but instead may reason about any first-order class + inductive definition. + +.. opt:: Firstorder Solver @tactic + :name: Firstorder Solver + + The default tactic used by :tacn:`firstorder` when no rule applies is + :g:`auto with core`, it can be reset locally or globally using this option. + + .. cmd:: Print Firstorder Solver + + Prints the default tactic used by :tacn:`firstorder` when no rule applies. + +.. tacv:: firstorder @tactic + + Tries to solve the goal with :n:`@tactic` when no logical rule may apply. + +.. tacv:: firstorder using {+ @qualid} + + .. deprecated:: 8.3 + + Use the syntax below instead (with commas). + +.. tacv:: firstorder using {+, @qualid} + + Adds lemmas :n:`{+, @qualid}` to the proof-search environment. If :n:`@qualid` + refers to an inductive type, it is the collection of its constructors which are + added to the proof-search environment. + +.. tacv:: firstorder with {+ @ident} + + Adds lemmas from :tacn:`auto` hint bases :n:`{+ @ident}` to the proof-search + environment. + +.. tacv:: firstorder @tactic using {+, @qualid} with {+ @ident} + + This combines the effects of the different variants of :tacn:`firstorder`. + +.. opt:: Firstorder Depth @natural + :name: Firstorder Depth + + This option controls the proof-search depth bound. + +.. tacn:: congruence + :name: congruence + + The tactic :tacn:`congruence`, by Pierre Corbineau, implements the standard + Nelson and Oppen congruence closure algorithm, which is a decision procedure + for ground equalities with uninterpreted symbols. It also includes + constructor theory (see :tacn:`injection` and :tacn:`discriminate`). If the goal + is a non-quantified equality, congruence tries to prove it with non-quantified + equalities in the context. Otherwise it tries to infer a discriminable equality + from those in the context. Alternatively, congruence tries to prove that a + hypothesis is equal to the goal or to the negation of another hypothesis. + + :tacn:`congruence` is also able to take advantage of hypotheses stating + quantified equalities, but you have to provide a bound for the number of extra + equalities generated that way. Please note that one of the sides of the + equality must contain all the quantified variables in order for congruence to + match against it. + +.. example:: + + .. coqtop:: reset all + + Theorem T (A:Type) (f:A -> A) (g: A -> A -> A) a b: a=(f a) -> (g b (f a))=(f (f a)) -> (g a b)=(f (g b a)) -> (g a b)=a. + intros. + congruence. + Qed. + + Theorem inj (A:Type) (f:A -> A * A) (a c d: A) : f = pair a -> Some (f c) = Some (f d) -> c=d. + intros. + congruence. + Qed. + +.. tacv:: congruence @natural + + Tries to add at most :token:`natural` instances of hypotheses stating quantified equalities + to the problem in order to solve it. A bigger value of :token:`natural` does not make + success slower, only failure. You might consider adding some lemmas as + hypotheses using assert in order for :tacn:`congruence` to use them. + +.. tacv:: congruence with {+ @term} + :name: congruence with + + Adds :n:`{+ @term}` to the pool of terms used by :tacn:`congruence`. This helps + in case you have partially applied constructors in your goal. + +.. exn:: I don’t know how to handle dependent equality. + + The decision procedure managed to find a proof of the goal or of a + discriminable equality but this proof could not be built in |Coq| because of + dependently-typed functions. + +.. exn:: Goal is solvable by congruence but some arguments are missing. Try congruence with {+ @term}, replacing metavariables by arbitrary terms. + + The decision procedure could solve the goal with the provision that additional + arguments are supplied for some partially applied constructors. Any term of an + appropriate type will allow the tactic to successfully solve the goal. Those + additional arguments can be given to congruence by filling in the holes in the + terms given in the error message, using the :tacn:`congruence with` variant described above. + +.. flag:: Congruence Verbose + + This flag makes :tacn:`congruence` print debug information. + +.. tacn:: btauto + :name: btauto + + The tactic :tacn:`btauto` implements a reflexive solver for boolean + tautologies. It solves goals of the form :g:`t = u` where `t` and `u` are + constructed over the following grammar: + + .. prodn:: + btauto_term ::= @ident + | true + | false + | orb @btauto_term @btauto_term + | andb @btauto_term @btauto_term + | xorb @btauto_term @btauto_term + | negb @btauto_term + | if @btauto_term then @btauto_term else @btauto_term + + Whenever the formula supplied is not a tautology, it also provides a + counter-example. + + Internally, it uses a system very similar to the one of the ring + tactic. + + Note that this tactic is only available after a ``Require Import Btauto``. + + .. exn:: Cannot recognize a boolean equality. + + The goal is not of the form :g:`t = u`. Especially note that :tacn:`btauto` + doesn't introduce variables into the context on its own. + +.. tacv:: field + field_simplify {* @term} + field_simplify_eq + + The field tactic is built on the same ideas as ring: this is a + reflexive tactic that solves or simplifies equations in a field + structure. The main idea is to reduce a field expression (which is an + extension of ring expressions with the inverse and division + operations) to a fraction made of two polynomial expressions. + + Tactic :n:`field` is used to solve subgoals, whereas :n:`field_simplify {+ @term}` + replaces the provided terms by their reduced fraction. + :n:`field_simplify_eq` applies when the conclusion is an equation: it + simplifies both hand sides and multiplies so as to cancel + denominators. So it produces an equation without division nor inverse. + + All of these 3 tactics may generate a subgoal in order to prove that + denominators are different from zero. + + See :ref:`Theringandfieldtacticfamilies` for more information on the tactic and how to + declare new field structures. All declared field structures can be + printed with the Print Fields command. + +.. example:: + + .. coqtop:: reset all + + Require Import Reals. + Goal forall x y:R, + (x * y > 0)%R -> + (x * (1 / x + x / (x + y)))%R = + ((- 1 / y) * y * (- x * (x / (x + y)) - 1))%R. + + intros; field. + +.. seealso:: + + File plugins/ring/RealField.v for an example of instantiation, + theory theories/Reals for many examples of use of field. |