to access context sensitive information within break-rewrite
Major Section:  BREAK-REWRITE

(brr@ :target)      ; the term being rewritten
(brr@ :unify-subst) ; the unifying substitution

General Form: (brr@ :symbol)

where :symbol is one of the following keywords. Those marked with * probably require an implementor's knowledge of the system to use effectively. They are supported but not well documented. More is said on this topic following the table.
:symbol             (brr@ :symbol)
-------             ---------------------

:target the term to be rewritten. This term is an instantiation of the left-hand side of the conclusion of the rewrite-rule being broken. This term is in translated form! Thus, if you are expecting (equal x nil) -- and your expectation is almost right -- you will see (equal x 'nil); similarly, instead of (cadr a) you will see (car (cdr a)). In translated forms, all constants are quoted (even nil, t, strings and numbers) and all macros are expanded.

:unify-subst the substitution that, when applied to :target, produces the left-hand side of the rule being broken. This substitution is an alist pairing variable symbols to translated (!) terms.

:wonp t or nil indicating whether the rune was successfully applied. (brr@ :wonp) returns nil if evaluated before :EVALing the rule.

:rewritten-rhs the result of successfully applying the rule or else nil if (brr@ :wonp) is nil. The result of successfully applying the rule is always a translated (!) term and is never nil.

:failure-reason some non-nil lisp object indicating why the rule was not applied or else nil. Before the rule is :EVALed, (brr@ :failure-reason) is nil. After :EVALing the rule, (brr@ :failure-reason) is nil if (brr@ :wonp) is t. Rather than document the various non-nil objects returned as the failure reason, we encourage you simply to evaluate (brr@ :failure-reason) in the contexts of interest. Alternatively, study the ACL2 function tilde-@- failure-reason-phrase.

:lemma * the rewrite rule being broken. For example, (access rewrite-rule (brr@ :lemma) :lhs) will return the left-hand side of the conclusion of the rule.

:type-alist * a display of the type-alist governing :target. Elements on the displayed list are of the form (term type), where term is a term and type describes information about term assumed to hold in the current context. The type-alist may be used to determine the current assumptions, e.g., whether A is a CONSP.

:ancestors * a stack of frames indicating the backchain history of the current context. The theorem prover is in the process of trying to establish each hypothesis in this stack. Thus, the negation of each hypothesis can be assumed false. Each frame also records the rules on behalf of which this backchaining is being done and the weight (function symbol count) of the hypothesis. All three items are involved in the heuristic for preventing infinite backchaining.

:gstack * the current goal stack. The gstack is maintained by rewrite and is the data structure printed as the current ``path.'' Thus, any information derivable from the :path brr command is derivable from gstack. For example, from gstack one might determine that the current term is the second hypothesis of a certain rewrite rule.

In general brr@-expressions are used in break conditions, the expressions that determine whether interactive breaks occur when monitored runes are applied. See monitor. For example, you might want to break only those attempts in which one particular term is being rewritten or only those attempts in which the binding for the variable a is known to be a consp. Such conditions can be expressed using ACL2 system functions and the information provided by brr@. Unfortunately, digging some of this information out of the internal data structures may be awkward or may, at least, require intimate knowledge of the system functions. But since conditional expressions may employ arbitrary functions and macros, we anticipate that a set of convenient primitives will gradually evolve within the ACL2 community. It is to encourage this evolution that brr@ provides access to the *'d data.