NOTE-2-6-RULES.html -- ACL2 Version 6.3

### NOTE-2-6-RULES

ACL2 Version 2.6 Notes on Changes in Rules and Constants

    Major Section:  NOTE-2-6

The following symbols have been added to the list constant `*common-lisp-specials-and-constants*`: `REPLACE`, `FILL`, `CHARACTER`, `=`, `BREAK`, and `PRIN1`. This was done in support of ports to Allegro 6.0 and Windows platforms (see note-2-6-new-functionality).

The list of symbols in `*acl2-exports*` has been modified, for example to include `show-accumulated-persistence` and the legal arguments to `set-inhibit-output-lst`.

Functions `zp` and `zip` are now handled slightly differently. They are are now disabled, but each comes with a `:``rewrite` rule that allows their expansion on non-variable terms, and also with a `:``compound-recognizer` rule that avoids the need for opening up these functions when applied to variables. The resulting behavior should be very similar to the behavior of previous versions, except that case splits will be avoided when these functions are applied to variables.

Function `standard-string-alistp` replaces function `string-alistp`. For further discussion, see note-2-6-guards.

Rules of class `:``rewrite` whose conclusion is a term of the form `(equal lhs rhs)` have always been stored in the expected way: `lhs` rewrites to `rhs`. This way of storing `:rewrite` rules has been extended to allow `=`, `eq`, or `eql` in place of `equal`.

Rewrite rule `nth-update-nth`, in source file `axioms.lisp`, has been strengthened.

A new rewrite rule `equal-constant-+` has been added to the book `arithmetic/equalities`. This should generally be a beneficial change, but existing proofs involving the arithmetic books could conceivably be affected.

Function `symbol-package-name` and constant `*main-lisp-package-name*` have undergone small changes. This change should rarely be noticed by users and is discussed elsewhere; see note-2-6-system.

We mention here that proofs involving stobjs may need to be modified because of changes in auxiliary functions generated by `defstobj`. (These changes were made in support of a new resizing capability, mentioned elsewhere in these release notes; see note-2-6-new-functionality.

In the distributed book directory `books/arithmetic/`, the book `rationals-with-axioms-proved.lisp` has been renamed `rationals.lisp`.

(ACL2(r) only) Rewrite rules `realp-+`, `realp-*`, `realp-unary--`, and `realp-unary-/` have been added in analogy to existing rules `rationalp-+`, `rationalp-*`, `rationalp-unary--`, and `rationalp-unary-/`. Thanks to Jun Sawada for suggesting this change.

The definition of `aref1` has been modified slightly. Previously, if `*my-a*` were an array then `(aref1 'some-name *my-a* :header)` would evaluate to the `cdr` of the `header` of `*my-a*` rather than to its `default`. See arrays.

Changes have been made in the `ihs` books, based on suggestions from Jun Sawada, that support its use with ACL2(r) (see real). The primary change is to replace calls of `rationalp` with calls of `real/rationalp`, which should have no effect on users of standard ACL2.