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

`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)`

. 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
`: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)`

.