DEFINITION

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

See rule-classes for a general discussion of rule classes and how they are used to build rules from formulas. An example `:``corollary` formula from which a `:definition` rule might be built is:

```Example:
(implies (true-listp x)
(equal (len x)
(if (null x)
0
(if (null (cdr x))
1
(+ 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 `hyp`s 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)`. (The user can obtain some control over this analysis by setting the default ruler-extenders; see ruler-extenders.) 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.

By default, a `:definition` rule establishes the so-called ``body'' of a function. The body is used by `:expand` hints, and it is also used heuristically by the theorem prover's preprocessing (the initial simplification using ``simple'' rules that is controlled by the `preprocess` symbol in `:do-not` hints), induction analysis, and the determination for when to warn about non-recursive functions in rules. The body is also used by some heuristics involving whether a function is recursively defined, and by the `expand`, `x`, and `x-dumb` commands of the proof-checker.

See rule-classes for a discussion of the optional field `:install-body` of `:definition` rules, which controls whether a `:definition` rule is used as described in the paragraph above. Note that even if `:install-body nil` is supplied, the rewriter will still rewrite with the `:definition` rule; in that case, ACL2 just won't install a new body for the top function symbol of the left-hand side of the rule, which for example affects the application of `:expand` hints as described in the preceding paragraph. Also see set-body and see show-bodies for how to change the body of a function symbol.

Note only that if you prove a definition rule for function `foo`, say, `foo-new-def`, you will need to refer to that definition as `foo-new-def` or as `(:DEFINITION foo-new-def)`. That is because a `:definition` rule does not change the meaning of the symbol `foo` for `:use` hints, nor does it change the meaning of the symbol `foo` in theory expressions; see theories, in particular the discussion there of runic designators. Similarly `:``pe` `foo` and `:``pf` `foo` will still show the original definition of `foo`.

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)`.