Major Section: MISCELLANEOUS

Examples: (IMPLIES (AND (RATIONALP LHS) (RATIONALP RHS) (BIND-FREE (FIND-MATCH-IN-PLUS-NESTS LHS RHS) (X))) (EQUAL (EQUAL LHS RHS) (EQUAL (+ (- X) LHS) (+ (- X) RHS)))) (IMPLIES (AND (BIND-FREE (FIND-RATIONAL-MATCH-IN-TIMES-NESTS LHS RHS MFC STATE) (X)) (RATIONALP X) (CASE-SPLIT (NOT (EQUAL X 0)))) (EQUAL (< LHS RHS) (IF (< 0 X) (< (* (/ X) LHS) (* (/ X) RHS)) (< (* (/ X) RHS) (* (/ X) LHS)))))

### BIND-FREE-EXAMPLES -- examples pertaining to

`bind-free`

hypotheses

General Forms:

(BIND-FREE term var-list) (BIND-FREE term t) (BIND-FREE term)A rule which uses a

`bind-free`

hypothesis has similarities to both a rule
which uses a `syntaxp`

hypothesis and to a `:`

`meta`

rule.
`Bind-free`

is like `syntaxp`

, in that it logically always returns
`t`

but may affect the application of a `:`

`rewrite`

,
`:`

`definition`

, or `:`

`linear`

rule when it is called at the
top-level of a hypothesis. It is like a `:`

`meta`

rule, in that it
allows the user to perform transformations of terms under progammatic
control.Note that a `bind-free`

hypothesis does not, in general, deal with the
meaning or semantics or values of the terms, but rather with their syntactic
forms. Before attempting to write a rule which uses `bind-free`

, the user
should be familiar with `syntaxp`

and the internal form that ACL2 uses
for terms. This internal form is similar to what the user sees, but there
are subtle and important differences. `Trans`

can be used to view this
internal form.

Just as for a `syntaxp`

hypothesis, there are two basic types of
`bind-free`

hypotheses. The simpler type of `bind-free`

hypothesis may
be used as the nth hypothesis in a `:`

`rewrite`

, `:`

`definition`

,
or `:`

`linear`

rule whose `:`

`corollary`

is
`(implies (and hyp1 ... hypn ... hypk) (equiv lhs rhs))`

provided `term`

is a term, `term`

contains at least one variable, and every variable
occuring freely in `term`

occurs freely in `lhs`

or in some `hypi`

,
`i<n`

. In addition, `term`

must not use any stobjs. Later below we will
describe the second type, an *extended* `bind-free`

hypothesis, which
may use `state`

. Whether simple or extended, a `bind-free`

hypothesis
may return an alist that binds free variables, as explained below, or it may
return a list of such alists. We focus on the first of these cases: return
of a single binding alist. We conclude our discussion with a section that
covers the other case: return of a list of alists.

We begin our description of `bind-free`

by examining the first example
above in some detail.

We wish to write a rule which will cancel ``like'' addends from both sides of an equality. Clearly, one could write a series of rules such as

(DEFTHM THE-HARD-WAY-2-1 (EQUAL (EQUAL (+ A X B) (+ X C)) (EQUAL (+ A B) (FIX C))))

with one rule for each combination of positions the matching addends might be found in (if one knew before-hand the maximum number of addends that would appear in a sum). But there is a better way. (In what follows, we assume the presence of an appropriate set of rules for simplifying sums.)

Consider the following definitions and theorem:

(DEFUN INTERSECTION-EQUAL (X Y) (COND ((ENDP X) NIL) ((MEMBER-EQUAL (CAR X) Y) (CONS (CAR X) (INTERSECTION-EQUAL (CDR X) Y))) (T (INTERSECTION-EQUAL (CDR X) Y)))) (DEFUN PLUS-LEAVES (TERM) (IF (EQ (FN-SYMB TERM) 'BINARY-+) (CONS (FARGN TERM 1) (PLUS-LEAVES (FARGN TERM 2))) (LIST TERM))) (DEFUN FIND-MATCH-IN-PLUS-NESTS (LHS RHS) (IF (AND (EQ (FN-SYMB LHS) 'BINARY-+) (EQ (FN-SYMB RHS) 'BINARY-+)) (LET ((COMMON-ADDENDS (INTERSECTION-EQUAL (PLUS-LEAVES LHS) (PLUS-LEAVES RHS)))) (IF COMMON-ADDENDS (LIST (CONS 'X (CAR COMMON-ADDENDS))) NIL)) NIL)) (DEFTHM CANCEL-MATCHING-ADDENDS-EQUAL (IMPLIES (AND (RATIONALP LHS) (RATIONALP RHS) (BIND-FREE (FIND-MATCH-IN-PLUS-NESTS LHS RHS) (X))) (EQUAL (EQUAL LHS RHS) (EQUAL (+ (- X) LHS) (+ (- X) RHS)))))

How is this rule applied to the following term?

(equal (+ 3 (expt a n) (foo a c)) (+ (bar b) (expt a n)))As mentioned above, the internal form of an ACL2 term is not always what one sees printed out by ACL2. In this case, by using

`:`

`trans`

one can
see that the term is stored internally as
(equal (binary-+ '3 (binary-+ (expt a n) (foo a c))) (binary-+ (bar b) (expt a n))).

When ACL2 attempts to apply `cancel-matching-addends-equal`

to the term
under discussion, it first forms a substitution that instantiates the
left-hand side of the conclusion so that it is identical to the target term.
This substitution is kept track of by the substitution alist:

((LHS . (binary-+ '3 (binary-+ (expt a n) (foo a c)))) (RHS . (binary-+ (bar b) (expt a n)))).ACL2 then attempts to relieve the hypotheses in the order they were given. Ordinarily this means that we instantiate each hypothesis with our substitution and then attempt to rewrite the resulting instance to true. Thus, in order to relieve the first hypothesis, we rewrite:

(RATIONALP (binary-+ '3 (binary-+ (expt a n) (foo a c)))).Let us assume that the first two hypotheses rewrite to

`t`

. How do we
relieve the `bind-free`

hypothesis? Just as for a `syntaxp`

hypothesis, ACL2 evaluates `(find-match-in-plus-nests lhs rhs)`

in an
environment where `lhs`

and `rhs`

are instantiated as determined by the
substitution. In this case we evaluate
(FIND-MATCH-IN-PLUS-NESTS '(binary-+ '3 (binary-+ (expt a n) (foo a c))) '(binary-+ (bar b) (expt a n))).Observe that, just as in the case of a

`syntaxp`

hypothesis, we
substitute the quotation of the variables bindings into the term to be
evaluated. See syntaxp for the reasons for this. The result of this
evaluation, `((X . (EXPT A N)))`

, is then used to extend the substitution
alist:
((X . (EXPT A N)) (LHS . (binary-+ '3 (binary-+ (expt a n) (foo a c)))) (RHS . (binary-+ (bar b) (expt a n)))),and this extended substitution determines

`cancel-matching-addends-equal`

's
result:
(EQUAL (+ (- X) LHS) (+ (- X) RHS)) ==> (EQUAL (+ (- (EXPT A N)) 3 (EXPT A N) (FOO A C)) (+ (- (EXPT A N)) (BAR B) (EXPT A N))).Question: What is the internal form of this result?

Hint: Use

`:`

`trans`

.When this rule fires, it adds the negation of a common term to both sides of
the equality by selecting a binding for the otherwise-free variable `x`

,
under programmatic control. Note that other mechanisms such as the binding
of free-variables may also extend the substitution alist.

Just as for a `syntaxp`

test, a `bind-free`

form signals failure by
returning `nil`

. However, while a `syntaxp`

test signals success by
returning true, a `bind-free`

form signals success by returning an alist
which is used to extend the current substitution alist. Because of this use
of the alist, there are several restrictions on it -- in particular the
alist must only bind variables, these variables must not be already bound by
the substitution alist, and the variables must be bound to ACL2 terms. If
`term`

returns an alist and the alist meets these restrictions, we append
the alist to the substitution alist and use the result as the new current
substitution alist. This new current substitution alist is then used when we
attempt to relieve the next hypothesis or, if there are no more, instantiate
the right hand side of the rule.

There is also a second, optional, `var-list`

argument to a `bind-free`

hypothesis. If provided, it must be either `t`

or a list of variables. If
it is not provided, it defaults to `t`

. If it is a list of variables, this
second argument is used to place a further restriction on the possible values
of the alist to be returned by `term`

: any variables bound in the alist
must be present in the list of variables. We strongly recommend the use of
this list of variables, as it allows some consistency checks to be performed
at the time of the rule's admittance which are not possible otherwise.

An extended `bind-free`

hypothesis is similar to the simple type described
above, but it uses two additional variables, `mfc`

and `state`

, which
must not be bound by the left hand side or an earlier hypothesis of the rule.
They must be the last two variables mentioned by `term`

: first `mfc`

,
then `state`

. These two variables give access to the functions
`mfc-`

xxx; see extended-metafunctions. As described there, `mfc`

is
bound to the so-called metafunction-context and `state`

to ACL2's
`state`

. See bind-free-examples for examples of the use of these
extended `bind-free`

hypotheses.

**SECTION**: Returning a list of alists.

As promised above, we conclude with a discussion of the case that evaluation
of the `bind-free`

term produces a list of alists, `x`

, rather than a
single alist. In this case each member `b`

of `x`

is considered in turn,
starting with the first and proceeding through the list. Each such `b`

is
handled exactly as discussed above, as though it were the result of
evaluating the `bind-free`

term. Thus, each `b`

extends the current
variable binding alist, and all remaining hypotheses are then relieved, as
though `b`

had been the value obtained by evaluating the `bind-free`

term. As soon as one such `b`

leads to successful relieving of all
remaining hypotheses, the process of relieving hypotheses concludes, so no
further members of `x`

are considered.

We illustrate with a simple pedagogical example. First introduce functions
`p1`

and `p2`

such that a rewrite rule specifies that `p2`

implies
`p1`

, but with a free variable.

(defstub p1 (x) t) (defstub p2 (x y) t) (defaxiom p2-implies-p1 (implies (p2 x y) (p1 x)))If we add the following axiom, then

`(p1 x)`

follows logically for all
`x`

.
(defaxiom p2-instance (p2 v (cons v 4)))Unfortunately, evaluation of

`(thm (p1 a))`

fails, because ACL2 fails to
bind the free variable `y`

in order to apply the rule `p2-instance`

.Let's define a function that produces a list of alists, each binding the
variable `y`

. Of course, we know that only the middle one below is
necessary in this simple example. In more complex examples, one might use
heuristics to construct such a list of alists.

(defun my-alists (x) (list (list (cons 'y (fcons-term* 'cons x ''3))) (list (cons 'y (fcons-term* 'cons x ''4))) (list (cons 'y (fcons-term* 'cons x ''5)))))The following rewrite rule uses

`bind-free`

to return a list of candidate
alists binding `y`

.
(defthm p2-implies-p1-better (implies (and (bind-free (my-alists x) (y)) ; the second argument, (y), is optional (p2 x y)) (p1 x)))Now the proof succeeds for

`(thm (p1 a))`

. Why? When ACL2 applies the
`rewrite`

rule `p2-implies-p1-better`

, it evaluates `my-alists`

, as we
can see from the following trace, to bind `y`

in three different
alists.
ACL2 !>(thm (p1 a)) 1> (ACL2_*1*_ACL2::MY-ALISTS A) <1 (ACL2_*1*_ACL2::MY-ALISTS (((Y CONS A '3)) ((Y CONS A '4)) ((Y CONS A '5)))) Q.E.D.The first alist, binding

`y`

to `(cons a '3)`

, fails to allow the
hypothesis `(p2 x y)`

to be proved. But the next binding of `y`

, to
`(cons a '4)`

, succeeds: then the current binding alist is
`((x . a) (y . (cons a '4)))`

, for which the hypothesis `(p2 x y)`

rewrites to true using the rewrite rule `p2-instance`

.