generate markers to indicate sources of guard proof obligations
Major Section:  GUARD

ACL2 guard verification (see guard) is often best avoided by beginning users of ACL2. When guard verification is employed, it can generate numerous goals, some of which may not be theorems if the definition being processed has bugs. It can be difficult to find such bugs. This documentation topic explains a capability provided by ACL2 to help find such bugs.

(Extra-info x y) always returns t by definition. However, if guard verification takes place with a non-nil setting of guard-debug (see below), then the goals generated for guard verification can include hypotheses that are calls of extra-info. Typically, such a hypothesis is of the following form:

(extra-info '(:guard (:body F))
            '(G ARG1 ... ARGk))
The above form indicates that the goal in which it occurs was generated to verify that the guard of the function F is satisfied by the arguments ARG1 through ARGk, where the term (G ARG1 ... ARGk) occurs in the body of the function F whose guard verification is in progress. If however the above call of G occurs in the guard of F instead of the body of F, then :body is replaced by :guard above:
(extra-info '(:guard (:guard F))
            '(G ARG1 ... ARGk))
If the same proof obligation (goal clause) arises from more than one occurrence of the same call, then a single goal will be generated, which has several extra-info hypotheses added to show the multiple sources of that proof obligation.

All rules (see rune) associated with extra-info are disabled by default, so that hypotheses of the form described above are not simplified to t. These hypotheses are intended to ride along for free: you can generally expect the proof to have the same structure whether or not the :guard-debug option is supplied, though on rare occasions the proofs may diverge.

It remains to explain the notion of a guard-debug setting of t, that is, to explain how to obtain the additional hypotheses described above. If guards are being verified during processing of a defun event (whether or not inside a call of mutual-recursion), then one specifies :guard-debug t in an xargs declaration of a declare form; see xargs. If however the guard verification is on behalf of a verify-guards call, whether for a definition or a theorem, then one specifies the keyword argument :guard-debug t.