(** * Hoare2: Hoare Logic, Part II *) Set Warnings "-notation-overridden,-parsing,-deprecated-hint-without-locality". Set Warnings "-intuition-auto-with-star". From Coq Require Import Strings.String. From PLF Require Import Maps. From Coq Require Import Bool.Bool. From Coq Require Import Arith.Arith. From Coq Require Import Arith.EqNat. From Coq Require Import Arith.PeanoNat. Import Nat. From Coq Require Import Lia. From PLF Require Export Imp. From PLF Require Import Hoare. Set Default Goal Selector "!". Definition FILL_IN_HERE := <{True}>. (* QUIZ On a piece of paper, write down a specification (as a Hoare triple) for the following program: X := 2; Y := X + X *) (* QUIZ Write down a (useful) specification for the following program: X := X + 1; Y := X + 1 *) (* QUIZ Write down a (useful) specification for the following program: if X <= Y then skip else Z := X; X := Y; Y := Z end *) (* QUIZ Write down a (useful) specification for the following program: X := m; Y := X + X *) (* QUIZ Write down a (useful) specification for the following program: X := m; Z := 0; while X <> 0 do X := X - 2; Z := Z + 1 end *) (* ################################################################# *) (** * Decorated Programs *) (** The beauty of Hoare Logic is that it is _structure-guided_: the structure of proofs exactly follows the structure of programs. We can record the essential ideas of a Hoare-logic proof -- omitting low-level calculational details -- by "decorating" a program with appropriate assertions on each of its commands. Such a _decorated program_ carries within itself an argument for its own correctness. *) (** For example, consider the program: X := m; Z := p; while X <> 0 do Z := Z - 1; X := X - 1 end *) (** Here is one possible specification for this program, in the form of a Hoare triple: {{ True }} X := m; Z := p; while X <> 0 do Z := Z - 1; X := X - 1 end {{ Z = p - m }} *) (** Here is a decorated version of this program, embodying a proof of this specification: {{ True }} ->> {{ m = m }} X := m {{ X = m }} ->> {{ X = m /\ p = p }}; Z := p; {{ X = m /\ Z = p }} ->> {{ Z - X = p - m }} while X <> 0 do {{ Z - X = p - m /\ X <> 0 }} ->> {{ (Z - 1) - (X - 1) = p - m }} Z := Z - 1 {{ Z - (X - 1) = p - m }}; X := X - 1 {{ Z - X = p - m }} end {{ Z - X = p - m /\ ~ (X <> 0) }} ->> {{ Z = p - m }} *) (** Concretely, a decorated program consists of the program's text interleaved with assertions (sometimes multiple assertions separated by implications). *) (** A decorated program can be viewed as a compact representation of a proof in Hoare Logic: the assertions surrounding each command specify the Hoare triple to be proved for that part of the program using one of the Hoare Logic rules, and the structure of the program itself shows how to assemble all these individual steps into a proof for the whole program. *) (* ================================================================= *) (** ** Example: Swapping *) (** Consider the following program, which swaps the values of two variables using addition and subtraction (instead of by assigning to a temporary variable). X := X + Y; Y := X - Y; X := X - Y We can give a proof, in the form of decorations, that this program is correct -- i.e., it really swaps [X] and [Y] -- as follows. *) (* WORK IN CLASS *) (* ================================================================= *) (** ** Example: Simple Conditionals *) (** Here's a simple program using conditionals, along with a possible specification: {{ True }} if X <= Y then Z := Y - X else Z := X - Y end {{ Z + X = Y \/ Z + Y = X }} Let's turn it into a decorated program... *) (* WORK IN CLASS *) (* ================================================================= *) (** ** Example: Reduce to Zero *) (** Here is a very simple [while] loop with a simple specification: {{ True }} while (X <> 0) do X := X - 1 end {{ X = 0 }} *) (* WORK IN CLASS *) (* ================================================================= *) (** ** Example: Division *) (** Let's do one more example of simple reasoning about a loop. The following Imp program calculates the integer quotient and remainder of parameters [m] and [n]. X := m; Y := 0; while n <= X do X := X - n; Y := Y + 1 end; If we replace [m] and [n] by concrete numbers and execute the program, it will terminate with the variable [X] set to the remainder when [m] is divided by [n] and [Y] set to the quotient. Here's a possible specification: {{ True }} X := m; Y := 0; while n <= X do X := X - n; Y := Y + 1 end {{ n * Y + X = m /\ X < n }} *) (* WORK IN CLASS *) (* ================================================================= *) (** ** From Decorated Programs to Formal Proofs *) (** From an informal proof in the form of a decorated program, it is "easy in principle" to read off a formal proof using the Coq theorems corresponding to the Hoare Logic rules, but these proofs can be a bit long and fiddly. *) (** For example... *) Definition reduce_to_zero : com := <{ while X <> 0 do X := X - 1 end }>. Theorem reduce_to_zero_correct' : {{True}} reduce_to_zero {{X = 0}}. Proof. unfold reduce_to_zero. (* First we need to transform the postcondition so that hoare_while will apply. *) eapply hoare_consequence_post. - apply hoare_while. + (* Loop body preserves loop invariant *) (* Massage precondition so [hoare_asgn] applies *) eapply hoare_consequence_pre. * apply hoare_asgn. * (* Proving trivial implication (2) ->> (3) *) unfold assertion_sub, "->>". simpl. intros. exact I. - (* Loop invariant and negated guard imply post *) intros st [Inv GuardFalse]. unfold bassertion in GuardFalse. simpl in GuardFalse. rewrite not_true_iff_false in GuardFalse. rewrite negb_false_iff in GuardFalse. apply eqb_eq in GuardFalse. apply GuardFalse. Qed. (** A little more (OK, quite a bit more) tactic fanciness for helping deal with the boring parts of the process of proving assertions: *) Ltac verify_assertion := repeat split; simpl; unfold assert_implies; unfold ap in *; unfold ap2 in *; unfold bassertion in *; unfold beval in *; unfold aeval in *; unfold assertion_sub; intros; repeat (simpl in *; rewrite t_update_eq || (try rewrite t_update_neq; [| (intro X; inversion X; fail)])); simpl in *; repeat match goal with [H : _ /\ _ |- _] => destruct H end; repeat rewrite not_true_iff_false in *; repeat rewrite not_false_iff_true in *; repeat rewrite negb_true_iff in *; repeat rewrite negb_false_iff in *; repeat rewrite eqb_eq in *; repeat rewrite eqb_neq in *; repeat rewrite leb_iff in *; repeat rewrite leb_iff_conv in *; try subst; simpl in *; repeat match goal with [st : state |- _] => match goal with | [H : st _ = _ |- _] => rewrite -> H in *; clear H | [H : _ = st _ |- _] => rewrite <- H in *; clear H end end; try eauto; try lia. (** This makes it pretty easy to verify [reduce_to_zero]: *) Theorem reduce_to_zero_correct''' : {{True}} reduce_to_zero {{X = 0}}. Proof. unfold reduce_to_zero. eapply hoare_consequence_post. - apply hoare_while. + eapply hoare_consequence_pre. * apply hoare_asgn. * verify_assertion. - verify_assertion. Qed. (** This example shows that it is conceptually straightforward to read off the main elements of a formal proof from a decorated program. Indeed, the process is so straightforward that it can be automated, as we will see next. *) (* ################################################################# *) (** * Formal Decorated Programs *) (** With a little more work, we can _formalize_ the definition of well-formed decorated programs and _automate_ the boring mechanical steps in proving that the decorations are correct. *) (* ================================================================= *) (** ** Syntax *) (** The first thing we need to do is to formalize a variant of the syntax of Imp commands that includes embedded assertions, which we'll call "decorations." We call the new commands _decorated commands_, or [dcom]s. The choice of exactly where to put assertions in the definition of [dcom] is a bit subtle. The simplest thing to do would be to annotate every [dcom] with a precondition and postcondition -- something like this... *) Module DComFirstTry. Inductive dcom : Type := | DCSkip (P : Assertion) (* {{ P }} skip {{ P }} *) | DCSeq (P : Assertion) (d1 : dcom) (Q : Assertion) (d2 : dcom) (R : Assertion) (* {{ P }} d1 {{Q}}; d2 {{ R }} *) | DCAsgn (X : string) (a : aexp) (Q : Assertion) (* etc. *) | DCIf (P : Assertion) (b : bexp) (P1 : Assertion) (d1 : dcom) (P2 : Assertion) (d2 : dcom) (Q : Assertion) | DCWhile (P : Assertion) (b : bexp) (P1 : Assertion) (d : dcom) (P2 : Assertion) (Q : Assertion) | DCPre (P : Assertion) (d : dcom) | DCPost (d : dcom) (Q : Assertion). End DComFirstTry. (** But this would result in _very_ verbose decorated programs with a lot of repeated annotations: even a simple program like [skip;skip] would be decorated like this, {{P}} ({{P}} skip {{P}}) ; ({{P}} skip {{P}}) {{P}} with pre- and post-conditions around each [skip], plus identical pre- and post-conditions on the semicolon! *) (** In other words, we don't want both preconditions and postconditions on each command, because a sequence of two commands would contain redundant decorations--the postcondition of the first likely being the same as the precondition of the second. Instead, the formal syntax of decorated commands omits preconditions whenever possible, trying to embed just the postcondition. *) (** - The [skip] command, for example, is decorated only with its postcondition skip {{ Q }} on the assumption that the precondition will be provided by the context. We carry the same assumption through the other syntactic forms: each decorated command is assumed to carry its own postcondition within itself but take its precondition from its context in which it is used. *) (** - Sequences [d1 ; d2] need no additional decorations. Why? Because inside [d2] there will be a postcondition; this serves as the postcondition of [d1;d2]. Similarly, inside [d1] there will also be a postcondition, which additionally serves as the _precondition_ for [d2]. *) (** - An assignment [X := a] is decorated only with its postcondition: X := a {{ Q }} *) (** - A conditional [if b then d1 else d2] is decorated with a postcondition for the entire statement, as well as preconditions for each branch: if b then {{ P1 }} d1 else {{ P2 }} d2 end {{ Q }} *) (** - A loop [while b do d end] is decorated with its postcondition and a precondition for the body: while b do {{ P }} d end {{ Q }} The postcondition embedded in [d] serves as the loop invariant. *) (** - Implications [->>] can be added as decorations either for a precondition ->> {{ P }} d or for a postcondition d ->> {{ Q }} The former is waiting for another precondition to be supplied by the context (e.g., [{{ P'}} ->> {{ P }} d]); the latter relies on the postcondition already embedded in [d]. *) (** Putting this all together gives us the formal syntax of decorated commands: *) Inductive dcom : Type := | DCSkip (Q : Assertion) (* skip {{ Q }} *) | DCSeq (d1 d2 : dcom) (* d1 ; d2 *) | DCAsgn (X : string) (a : aexp) (Q : Assertion) (* X := a {{ Q }} *) | DCIf (b : bexp) (P1 : Assertion) (d1 : dcom) (P2 : Assertion) (d2 : dcom) (Q : Assertion) (* if b then {{ P1 }} d1 else {{ P2 }} d2 end {{ Q }} *) | DCWhile (b : bexp) (P : Assertion) (d : dcom) (Q : Assertion) (* while b do {{ P }} d end {{ Q }} *) | DCPre (P : Assertion) (d : dcom) (* ->> {{ P }} d *) | DCPost (d : dcom) (Q : Assertion) (* d ->> {{ Q }} *). (** (We then need to redefine all our Notations to get nice concrete syntax for [dcom].) *) (** To provide the initial precondition that goes at the very top of a decorated program, we introduce a new type [decorated]: *) Inductive decorated : Type := | Decorated : Assertion -> dcom -> decorated. (** To avoid clashing with the existing [Notation]s for ordinary commands, we introduce these notations in a new grammar scope called [dcom]. *) Declare Scope dcom_scope. Notation "'skip' {{ P }}" := (DCSkip P) (in custom com at level 0, P constr) : dcom_scope. Notation "l ':=' a {{ P }}" := (DCAsgn l a P) (in custom com at level 0, l constr at level 0, a custom com at level 85, P constr, no associativity) : dcom_scope. Notation "'while' b 'do' {{ Pbody }} d 'end' {{ Ppost }}" := (DCWhile b Pbody d Ppost) (in custom com at level 89, b custom com at level 99, Pbody constr, Ppost constr) : dcom_scope. Notation "'if' b 'then' {{ P }} d 'else' {{ P' }} d' 'end' {{ Q }}" := (DCIf b P d P' d' Q) (in custom com at level 89, b custom com at level 99, P constr, P' constr, Q constr) : dcom_scope. Notation "'->>' {{ P }} d" := (DCPre P d) (in custom com at level 12, right associativity, P constr) : dcom_scope. Notation "d '->>' {{ P }}" := (DCPost d P) (in custom com at level 10, right associativity, P constr) : dcom_scope. Notation " d ; d' " := (DCSeq d d') (in custom com at level 90, right associativity) : dcom_scope. Notation "{{ P }} d" := (Decorated P d) (in custom com at level 91, P constr) : dcom_scope. Local Open Scope dcom_scope. Example dec0 : dcom := <{ skip {{ True }} }>. Example dec1 : dcom := <{ while true do {{ True }} skip {{ True }} end {{ True }} }>. (** Recall that you can [Set Printing All] to see how all that notation is desugared. *) Set Printing All. Print dec1. Unset Printing All. (** An example [decorated] program that decrements [X] to [0]: *) Example dec_while : decorated := <{ {{ True }} while X <> 0 do {{ True /\ (X <> 0) }} X := X - 1 {{ True }} end {{ True /\ X = 0}} ->> {{ X = 0 }} }>. (** It is easy to go from a [dcom] to a [com] by erasing all annotations. *) Fixpoint erase (d : dcom) : com := match d with | DCSkip _ => CSkip | DCSeq d1 d2 => CSeq (erase d1) (erase d2) | DCAsgn X a _ => CAsgn X a | DCIf b _ d1 _ d2 _ => CIf b (erase d1) (erase d2) | DCWhile b _ d _ => CWhile b (erase d) | DCPre _ d => erase d | DCPost d _ => erase d end. Definition erase_d (dec : decorated) : com := match dec with | Decorated P d => erase d end. (** It is also straightforward to extract the precondition and postcondition from a decorated program. *) Definition precondition_from (dec : decorated) : Assertion := match dec with | Decorated P d => P end. Fixpoint post (d : dcom) : Assertion := match d with | DCSkip P => P | DCSeq _ d2 => post d2 | DCAsgn _ _ Q => Q | DCIf _ _ _ _ _ Q => Q | DCWhile _ _ _ Q => Q | DCPre _ d => post d | DCPost _ Q => Q end. Definition postcondition_from (dec : decorated) : Assertion := match dec with | Decorated P d => post d end. (** We can then express what it means for a decorated program to be correct as follows: *) Definition outer_triple_valid (dec : decorated) := {{precondition_from dec}} erase_d dec {{postcondition_from dec}}. (** For example: *) Example dec_while_triple_correct : outer_triple_valid dec_while = {{ True }} while X <> 0 do X := X - 1 end {{ X = 0 }}. Proof. reflexivity. Qed. (** The outer Hoare triple of a decorated program is just a [Prop]; thus, to show that it is _valid_, we need to produce a proof of this proposition. We will do this by extracting "proof obligations" from the decorations sprinkled through the program. These obligations are often called _verification conditions_, because they are the facts that must be verified to see that the decorations are locally consistent and thus constitute a proof of validity of the outer triple. *) (* ================================================================= *) (** ** Extracting Verification Conditions *) (** The function [verification_conditions] takes a decorated command [d] together with a precondition [P] and returns a _proposition_ that, if it can be proved, implies that the triple {{P}} erase d {{post d}} is valid. It does this by walking over [d] and generating a big conjunction that includes - local consistency checks for each form of command, plus - uses of [->>] to bridge the gap between the assertions found inside a decorated command and the assertions imposed by the precondition from its context; these uses correspond to applications of the consequence rule. _Local consistency_ is defined as follows... *) (** - The decorated command skip {{Q}} is locally consistent with respect to a precondition [P] if [P ->> Q]. *) (** - The sequential composition of [d1] and [d2] is locally consistent with respect to [P] if [d1] is locally consistent with respect to [P] and [d2] is locally consistent with respect to the postcondition of [d1]. - An assignment X := a {{Q}} is locally consistent with respect to a precondition [P] if: P ->> Q [X |-> a] *) (** - A conditional if b then {{P1}} d1 else {{P2}} d2 end {{Q}} is locally consistent with respect to precondition [P] if (1) [P /\ b ->> P1] (2) [P /\ ~b ->> P2] (3) [d1] is locally consistent with respect to [P1] (4) [d2] is locally consistent with respect to [P2] (5) [post d1 ->> Q] (6) [post d2 ->> Q] *) (** - A loop while b do {{Q}} d end {{R}} is locally consistent with respect to precondition [P] if: (1) [P ->> post d] (2) [post d /\ b ->> Q] (3) [post d /\ ~b ->> R] (4) [d] is locally consistent with respect to [Q] *) (** - A command with an extra assertion at the beginning --> {{Q}} d is locally consistent with respect to a precondition [P] if: (1) [P ->> Q] (1) [d] is locally consistent with respect to [Q] *) (** - A command with an extra assertion at the end d ->> {{Q}} is locally consistent with respect to a precondition [P] if: (1) [d] is locally consistent with respect to [P] (2) [post d ->> Q] *) (** With all this in mind, we can write is a _verification condition generator_ that takes a decorated command and reads off a proposition saying that all its decorations are locally consistent. Formally, since a decorated command is "waiting for its precondition" the main VC generator takes a [dcom] plus a given predondition as arguments. *) Fixpoint verification_conditions (P : Assertion) (d : dcom) : Prop := match d with | DCSkip Q => (P ->> Q) | DCSeq d1 d2 => verification_conditions P d1 /\ verification_conditions (post d1) d2 | DCAsgn X a Q => (P ->> Q [X |-> a]) | DCIf b P1 d1 P2 d2 Q => ((P /\ b) ->> P1)%assertion /\ ((P /\ ~ b) ->> P2)%assertion /\ (post d1 ->> Q) /\ (post d2 ->> Q) /\ verification_conditions P1 d1 /\ verification_conditions P2 d2 | DCWhile b Q d R => (* post d is both the loop invariant and the initial precondition *) (P ->> post d) /\ ((post d /\ b) ->> Q)%assertion /\ ((post d /\ ~ b) ->> R)%assertion /\ verification_conditions Q d | DCPre P' d => (P ->> P') /\ verification_conditions P' d | DCPost d Q => verification_conditions P d /\ (post d ->> Q) end. (** The following key theorem states that [verification_conditions] does its job correctly. Not surprisingly, each of the Hoare Logic rules gets used at some point in the proof. *) Theorem verification_correct : forall d P, verification_conditions P d -> {{P}} erase d {{post d}}. Proof. induction d; intros; simpl in *. - (* Skip *) eapply hoare_consequence_pre. + apply hoare_skip. + assumption. - (* Seq *) destruct H as [H1 H2]. eapply hoare_seq. + apply IHd2. apply H2. + apply IHd1. apply H1. - (* Asgn *) eapply hoare_consequence_pre. + apply hoare_asgn. + assumption. - (* If *) destruct H as [HPre1 [HPre2 [Hd1 [Hd2 [HThen HElse] ] ] ] ]. apply IHd1 in HThen. clear IHd1. apply IHd2 in HElse. clear IHd2. apply hoare_if. + eapply hoare_consequence; eauto. + eapply hoare_consequence; eauto. - (* While *) destruct H as [Hpre [Hbody1 [Hpost1 Hd] ] ]. eapply hoare_consequence; eauto. apply hoare_while. eapply hoare_consequence_pre; eauto. - (* Pre *) destruct H as [HP Hd]. eapply hoare_consequence_pre; eauto. - (* Post *) destruct H as [Hd HQ]. eapply hoare_consequence_post; eauto. Qed. (** Now that all the pieces are in place, we can define what it means to verify an entire program. *) Definition verification_conditions_from (dec : decorated) : Prop := match dec with | Decorated P d => verification_conditions P d end. (** This brings us to the main theorem of this section: *) Corollary verification_conditions_correct : forall dec, verification_conditions_from dec -> outer_triple_valid dec. Proof. intros [P d]. apply verification_correct. Qed. (* ================================================================= *) (** ** More Automation *) (** The propositions generated by [verification_conditions] are fairly big and contain many conjuncts that are essentially trivial. *) Eval simpl in verification_conditions_from dec_while. (* ==> ((fun _ : state => True) ->> (fun _ : state => True)) /\ ((fun st : state => True /\ negb (st X =? 0) = true) ->> (fun st : state => True /\ st X <> 0)) /\ ((fun st : state => True /\ negb (st X =? 0) <> true) ->> (fun st : state => True /\ st X = 0)) /\ (fun st : state => True /\ st X <> 0) ->> (fun _ : state => True) [X |-> X - 1]) /\ (fun st : state => True /\ st X = 0) ->> (fun st : state => st X = 0) : Prop *) (** Fortunately, our [verify_assertion] tactic can generally take care of most or all of them. *) Example vc_dec_while : verification_conditions_from dec_while. Proof. verify_assertion. Qed. (** To automate the overall process of verification, we can use [verification_correct] to extract the verification conditions, use [verify_assertion] to verify them as much as it can, and finally tidy up any remaining bits by hand. *) Ltac verify := intros; apply verification_correct; verify_assertion. (** Here's the final, formal proof that dec_while is correct. *) Theorem dec_while_correct : outer_triple_valid dec_while. Proof. verify. Qed. (* ################################################################# *) (** * Finding Loop Invariants *) (** Once the outer pre- and postcondition are chosen, the only creative part in verifying programs using Hoare Logic is finding the right loop invariants... *) (* ================================================================= *) (** ** Example: Slow Subtraction *) (** The following program subtracts the value of [X] from the value of [Y] by repeatedly decrementing both [X] and [Y]. We want to verify its correctness with respect to the pre- and postconditions shown: {{ X = m /\ Y = n }} while X <> 0 do Y := Y - 1; X := X - 1 end {{ Y = n - m }} *) (** To verify this program, we need to find an invariant [Inv] for the loop. As a first step we can leave [Inv] as an unknown and build a _skeleton_ for the proof by applying the rules for local consistency, working from the end of the program to the beginning, as usual, and without any thinking at all yet. *) (** This leads to the following skeleton: (1) {{ X = m /\ Y = n }} ->> (a) (2) {{ Inv }} while X <> 0 do (3) {{ Inv /\ X <> 0 }} ->> (c) (4) {{ Inv [X |-> X-1] [Y |-> Y-1] }} Y := Y - 1; (5) {{ Inv [X |-> X-1] }} X := X - 1 (6) {{ Inv }} end (7) {{ Inv /\ ~ (X <> 0) }} ->> (b) (8) {{ Y = n - m }} *) (** By examining this skeleton, we can see that any valid [Inv] will have to respect three conditions: - (a) it must be _weak_ enough to be implied by the loop's precondition, i.e., (1) must imply (2); - (b) it must be _strong_ enough to imply the program's postcondition, i.e., (7) must imply (8); - (c) it must be _preserved_ by a single iteration of the loop, assuming that the loop guard also evaluates to true, i.e., (3) must imply (4). *) (* WORK IN CLASS (by filling in the previous template) *) (* ================================================================= *) (** ** Example: Parity *) (** Here is a cute way of computing the parity of a value initially stored in [X], due to Daniel Cristofani. {{ X = m }} while 2 <= X do X := X - 2 end {{ X = parity m }} The [parity] function used in the specification is defined in Coq as follows: *) Fixpoint parity x := match x with | 0 => 0 | 1 => 1 | S (S x') => parity x' end. Definition parity_dec (m:nat) : decorated := <{ {{ X = m }} ->> {{ FILL_IN_HERE }} while 2 <= X do {{ FILL_IN_HERE }} ->> {{ FILL_IN_HERE }} X := X - 2 {{ FILL_IN_HERE }} end {{ FILL_IN_HERE }} ->> {{ X = parity m }} }>. (* ================================================================= *) (** ** Example: Finding Square Roots *) (** The following program computes the integer square root of [X] by naive iteration: {{ X=m }} Z := 0; while (Z+1)*(Z+1) <= X do Z := Z+1 end {{ Z*Z<=m /\ m<(Z+1)*(Z+1) }} *) (* WORK IN CLASS *) (* ================================================================= *) (** ** Example: Squaring *) (** Here is a program that squares [X] by repeated addition: {{ X = m }} Y := 0; Z := 0; while Y <> X do Z := Z + X; Y := Y + 1 end {{ Z = m*m }} *) (* WORK IN CLASS *) (** [] *) (* ################################################################# *) (** * Weakest Preconditions (Optional) *) (** A useless (though valid) Hoare triple: {{ False }} X := Y + 1 {{ X <= 5 }} A better precondition: {{ Y <= 4 /\ Z = 0 }} X := Y + 1 {{ X <= 5 }} The _best_ precondition: {{ Y <= 4 }} X := Y + 1 {{ X <= 5 }} *) (** Assertion [Y <= 4] is a _weakest precondition_ of command [X := Y + 1] with respect to postcondition [X <= 5]. Think of _weakest_ here as meaning "easiest to satisfy": a weakest precondition is one that as many states as possible can satisfy. *) (** [P] is a weakest precondition of command [c] for postcondition [Q] if - [P] is a precondition, that is, [{{P}} c {{Q}}]; and - [P] is at least as weak as all other preconditions, that is, if [{{P'}} c {{Q}}] then [P' ->> P]. *) Definition is_wp P c Q := {{P}} c {{Q}} /\ forall P', {{P'}} c {{Q}} -> (P' ->> P). (** **** Exercise: 1 star, standard, optional (wp) What are weakest preconditions of the following commands for the following postconditions? 1) {{ ? }} skip {{ X = 5 }} 2) {{ ? }} X := Y + Z {{ X = 5 }} 3) {{ ? }} X := Y {{ X = Y }} 4) {{ ? }} if X = 0 then Y := Z + 1 else Y := W + 2 end {{ Y = 5 }} 5) {{ ? }} X := 5 {{ X = 0 }} 6) {{ ? }} while true do X := 0 end {{ X = 0 }} *) (* FILL IN HERE [] *)