make a rule that acts like a function definition
Major Section:  RULE-CLASSES

(implies (true-listp x)
         (equal (len x)
                (if (null x)
                    (if (null (cdr x))
                        (+ 2 (len (cddr x)))))))

General Form: (implies hyp (equiv (fn a1 ... an) body))

where equiv is an equivalence relation and fn is a function symbol other than if, hide, force or case-split. Such rules allow ``alternative'' definitions of fn to be proved as theorems but used as definitions. These rules are not true ``definitions'' in the sense that they (a) cannot introduce new function symbols and (b) do not have to be terminating recursion schemes. They are just conditional rewrite rules that are controlled the same way we control recursive definitions. We call these ``definition rules'' or ``generalized definitions''.

Consider the general form above. Generalized definitions are stored among the :rewrite rules for the function ``defined,'' fn above, but the procedure for applying them is a little different. During rewriting, instances of (fn a1 ... an) are replaced by corresponding instances of body provided the hyps can be established as for a :rewrite rule and the result of rewriting body satisfies the criteria for function expansion. There are two primary criteria, either of which permits expansion. The first is that the ``recursive'' calls of fn in the rewritten body have arguments that already occur in the goal conjecture. The second is that the ``controlling'' arguments to fn are simpler in the rewritten body.

The notions of ``recursive call'' and ``controllers'' are complicated by the provisions for mutually recursive definitions. Consider a ``clique'' of mutually recursive definitions. Then a ``recursive call'' is a call to any function defined in the clique and an argument is a ``controller'' if it is involved in the measure that decreases in all recursive calls. These notions are precisely defined by the definitional principle and do not necessarily make sense in the context of generalized definitional equations as implemented here.

But because the heuristics governing the use of generalized definitions require these notions, it is generally up to the user to specify which calls in body are to be considered recursive and what the controlling arguments are. This information is specified in the :clique and :controller-alist fields of the :definition rule class.

The :clique field is the list of function symbols to be considered recursive calls of fn. In the case of a non-recursive definition, the :clique field is empty; in a singly recursive definition, it should consist of the singleton list containing fn; otherwise it should be a list of all of the functions in the mutually recursive clique with this definition of fn.

If the :clique field is not provided it defaults to nil if fn does not occur as a function symbol in body and it defaults to the singleton list containing fn otherwise. Thus, :clique must be supplied by the user only when the generalized definition rule is to be treated as one of several in a mutually recursive clique.

The :controller-alist is an alist that maps each function symbol in the :clique to a mask specifying which arguments are considered controllers. The mask for a given member of the clique, fn, must be a list of t's and nil's of length equal to the arity of fn. A t should be in each argument position that is considered a ``controller'' of the recursion. For a function admitted under the principle of definition, an argument controls the recursion if it is one of the arguments measured in the termination argument for the function. But in generalized definition rules, the user is free to designate any subset of the arguments as controllers. Failure to choose wisely may result in the ``infinite expansion'' of definitional rules but cannot render ACL2 unsound since the rule being misused is a theorem.

If the :controller-alist is omitted it can sometimes be defaulted automatically by the system. If the :clique is nil, the :controller-alist defaults to nil. If the :clique is a singleton containing fn, the :controller-alist defaults to the controller alist computed by (defun fn args body). If the :clique contains more than one function, the user must supply the :controller-alist specifying the controllers for each function in the clique. This is necessary since the system cannot determine and thus cannot analyze the other definitional equations to be included in the clique.

For example, suppose fn1 and fn2 have been defined one way and it is desired to make ``alternative'' mutually recursive definitions available to the rewriter. Then one would prove two theorems and store each as a :definition rule. These two theorems would exhibit equations ``defining'' fn1 and fn2 in terms of each other. No provision is here made for exhibiting these two equations as a system of equations. One is proved and then the other. It just so happens that the user intends them to be treated as mutually recursive definitions. To achieve this end, both :definition rules should specify the :clique (fn1 fn2) and should specify a suitable :controller-alist. If, for example, the new definition of fn1 is controlled by its first argument and the new definition of fn2 is controlled by its second and third (and they each take three arguments) then a suitable :controller-alist would be ((fn1 t nil nil) (fn2 nil t t)). The order of the pairs in the alist is unimportant, but there must be a pair for each function in the clique.

Inappropriate heuristic advice via :clique and :controller-alist can cause ``infinite expansion'' of generalized definitions, but cannot render ACL2 unsound.

Note that the actual definition of fn1 has the runic name (:definition fn1). The runic name of the alternative definition is (:definition lemma), where lemma is the name given to the event that created the generalized :definition rule. This allows theories to switch between various ``definitions'' of the functions.

The definitional principle, defun, actually adds :definition rules. Thus the handling of generalized definitions is exactly the same as for ``real'' definitions because no distinction is made in the implementation. Suppose (fn x y) is defun'd to be body. Note that defun (or defuns or mutual-recursion) can compute the clique for fn from the syntactic presentation and it can compute the controllers from the termination analysis. Provided the definition is admissible, defun adds the :definition rule (equal (fn x y) body).