SYNTAXP

attach a heuristic filter on a `:``rewrite`, `:``meta`, or `:``linear` rule
```Major Section:  MISCELLANEOUS
```

```Example:
Consider the :REWRITE rule created from

(IMPLIES (SYNTAXP (NOT (AND (CONSP X)
(EQ (CAR X) 'NORM))))
(EQUAL (LXD X)
(LXD (NORM X)))).
```
The `syntaxp` hypothesis in this rule will allow the rule to be applied to `(lxd (trn a b))` but will not allow it to be applied to `(lxd (norm a))`.

• SYNTAXP-EXAMPLES -- examples pertaining to syntaxp hypotheses

```General Form:
(SYNTAXP test)
```
`Syntaxp` always returns `t` and so may be added as a vacuous hypothesis. However, when relieving the hypothesis, the test ``inside'' the `syntaxp` form is actually treated as a meta-level proposition about the proposed instantiation of the rule's variables and that proposition must evaluate to true (non-`nil`) to ``establish'' the `syntaxp` hypothesis.

Note that the test of a `syntaxp` hypothesis does not, in general, deal with the meaning or semantics or values of the terms, but rather with their syntactic forms. In the example above, the `syntaxp` hypothesis allows the rule to be applied to every target of the form `(lxd u)`, provided `u` is not of the form `(norm v)`. Observe that without this syntactic restriction the rule above could loop producing a sequence of increasingly complex targets `(lxd a)`, `(lxd (norm a))`, `(lxd (norm (norm a)))`, etc. An intuitive reading of the rule might be ```norm` the argument of `lxd` unless it has already been `norm`ed.''

Note also that a `syntaxp` hypothesis deals with the syntactic form used internally by ACL2, rather than that seen by the user. In some cases these are the same, but there can be subtle differences with which the writer of a `syntaxp` hypothesis must be aware. You can use `:``trans` to display this internal representation.

There are two types of `syntaxp` hypotheses. The simpler type of `syntaxp` hypothesis may be used as the nth hypothesis in a `:``rewrite` or `:``linear` rule whose `:``corollary` is `(implies (and hyp1 ... hypn ... hypk) (equiv lhs rhs))` provided `test` is a term, `test` contains at least one variable, and every variable occuring freely in `test` occurs freely in `lhs` or in some `hypi`, `i<n`. In addition, `test` must not use any stobjs. The case of `:``meta` rules is similar to the above, except that it applies to the result of applying the hypothesis metafunction. (Later below we will describe the second type, an extended `syntaxp` hypothesis, which may use `state`.)

We illustrate the use of simple `syntaxp` hypotheses by slightly elaborating the example given above. Consider a `:``rewrite` rule whose `:``corollary` is:

```(IMPLIES (AND (RATIONALP X)
(SYNTAXP (NOT (AND (CONSP X)
(EQ (CAR X) 'NORM)))))
(EQUAL (LXD X)
(LXD (NORM X))))
```
How is this rule applied to `(lxd (trn a b))`? First, we form a substitution that instantiates the left-hand side of the conclusion of the rule so that it is identical to the target term. In the present case, the substitution replaces `x` with `(trn a b)`.
```(LXD X) ==> (LXD (trn a b)).
```
Then we backchain to establish the hypotheses, in order. Ordinarily this means that we instantiate each hypothesis with our substitution and then attempt to rewrite the resulting instance to true. Thus, in order to relieve the first hypothesis above, we rewrite
```(RATIONALP (trn a b)).
```
If this rewrites to true, we continue.

Of course, most users are aware of some exceptions to this general description of the way we relieve hypotheses. For example, if a hypothesis contains a ``free-variable'' -- one not bound by the current substitution -- we attempt to extend the substitution by searching for an instance of the hypothesis among known truths. See free-variables. `Force`d hypotheses are another exception to the general rule of how hypotheses are relieved.

Hypotheses marked with `syntaxp`, as in `(syntaxp test)`, are also exceptions. We instantiate such a hypothesis; but instead of rewriting the instantiated instance, we evaluate the instantiated `test`. More precisely, we evaluate `test` in an environment in which its variable symbols are bound to the quotations of the terms to which those variables are bound in the instantiating substitution. So in the case in point, we (in essence) evaluate

```(NOT (AND (CONSP '(trn a b)) (EQ (CAR '(trn a b)) 'NORM))).
```
This clearly evaluates to `t`. When a `syntaxp` test evaluates to true, we consider the `syntaxp` hypothesis to have been established; this is sound because logically `(syntaxp test)` is `t` regardless of `test`. If the test evaluates to `nil` (or fails to evaluate because of guard violations) we act as though we cannot establish the hypothesis and abandon the attempt to apply the rule; it is always sound to give up.

```(NOT (AND (CONSP '(trn a b)) (EQ (CAR '(trn a b)) 'NORM))).
```
When relieving the first hypothesis, `(RATIONALP X)`, we substituted `(trn a b)` for `X`; but when relieving the second hypothesis, `(SYNTAXP (NOT (AND (CONSP X) (EQ (CAR X) 'NORM))))`, we substituted the quotation of `(trn a b)` for `X`. Why the difference? Remember that in the first hypothesis we are talking about the value of `(trn a b)` -- is it rational -- while in the second one we are talking about its syntactic form. Remember also that Lisp, and hence ACL2, evaluates the arguments to a function before applying the function to the resulting values. Thus, we are asking ``Is the list `(trn a b)` a `consp` and if so, is its `car` the symbol `NORM`?'' The `quote`s on both `(trn a b)` and `NORM` are therefore necesary. One can verify this by defining `trn` to be, say `cons`, and then evaluating forms such as
```(AND (CONSP '(trn a b)) (EQ (CAR '(trn a b)) 'NORM))
(AND (CONSP (trn a b)) (EQ (CAR (trn a b)) NORM))
(AND (CONSP (trn 'a 'b)) (EQ (CAR (trn 'a 'b)) NORM))
(AND (CONSP '(trn a b)) (EQ '(CAR (trn a b)) ''NORM))
```
at the top-level ACL2 prompt.

See syntaxp-examples for more examples of the use of `syntaxp`.

An extended `syntaxp` hypothesis is similar to the simple type described above, but it uses two additional variables, `mfc` and `state`, which must not be bound by the left hand side or an earlier hypothesis of the rule. They must be the last two variables mentioned by `form`; first `mfc`, then `state`. These two variables give access to the functions `mfc-`xxx; see extended-metafunctions. As described there, `mfc` is bound to the so-called metafunction-context and `state` to ACL2's `state`. See syntaxp-examples for an example of the use of these extended `syntaxp` hypotheses.  