ACL2 Version 2.0 (July, 1997) Notes
Major Section:  RELEASE-NOTES

This is the first version of ACL2 released under the copyright of the University of Texas (UT). Future releases of ACL2 will be made from UT rather than Computational Logic, Inc. (CLI). Version 2.0 is just Version 1.9 as released by CLI, with a few bugs fixed.

A bug causing an infinite loop was fixed in functional instantiation. The bug manifested itself when two conditions occurred simultaneously: First, the functional substitution replaces a function symbol, e.g., FOO, with a LAMBDA expression containing a free variable (a variable not among in the LAMBDA formals). And, second, in one of the constraints being instantiated there is a call of the function symbol FOO within the scope of another LAMBDA expression. Unless you used such a functional substitution, this bug fix will not affect you.

Less important notes:

The implementation of PRINC$ was changed so that it was no longer sensitive to the external setting of *print-base* and other Common Lisp special variables.

Typographical errors were fixed in the documentation.


ACL2 Version 2.1 (December, 1997) Notes
Major Section:  RELEASE-NOTES

The identity function case-split has been added. It is similar to force but causes an immediate split of the top-level goal on whether the indicated hypothesis is true.

Less important notes:

Minor bugs in the documentation were fixed.


ACL2 Version 2.2 (August, 1998) Notes
Major Section:  RELEASE-NOTES

Important changes:

A bug was fixed in the compile command, :comp. The compiled code produced by :comp in previous versions could be wildly incorrect because of a confusion between the printer and the reader regarding what was the current Lisp *package*. This bug could manifest itself only if you used the :comp command to compile previously uncompiled functions while the current package was different from "ACL2". What happened in that situation depended upon what symbols were imported into your current package. The most likely behavior is that the compiler would break or complain or the resulting compiled code would call functions that did not exist.

There have been no other important changes to the code. However, this release contains some useful new books, notably those on the books subdirectories cli-misc and ihs. Both have README files. The ihs books provide support for integer hardware specifications. These books were crucial to Bishop Brock's successful modeling of the Motorola CAP. We thank Bishop for producing them and we thank all those who worked so hard to get these books released. We highly recommend the ihs books to those modeling ALUs and other arithmetic components of microprocessors or programming languages.

In previous versions of ACL2, the arithmetic books, found on books/arithmetic/, included the addition of several unproved axioms stating properties of the rationals that we believed could be derived from our ``official'' axioms but which we had not mechanically proved. The axioms were found in the book rationals-with-axioms.lisp, which was then used in the uppermost arithmetic books top.lisp and top-with-meta.lisp. John Cowles has now provided us with ACL2 proofs of those ``axioms'' and so in this release you will find both rationals-with-axioms.lisp and rationals-with-axioms-proved.lisp. The former is provided for compatibility's sake. The latter is identical but contains defthms where the former contains defaxioms. The top-most books have been rebuilt using ``-axioms-proved'' book. Thanks John.

Less important notes:

Bishop Brock found a bug in translated-acl2-unwind-protectp4. Jun Sawada reported a bug in linear arithmetic that caused us not to prove certain trivial theorems concluding with (not (equal i j)). We have fixed both.

We now prohibit definitions that call certain event commands such as DEFTHM and TABLE because our Common Lisp implementations of them differ from their ACL2 meanings (so that compiled books can be loaded correctly and efficiently).

Minor bugs in the documentation were fixed.


ACL2 Version 2.3 (October, 1998) Notes
Major Section:  RELEASE-NOTES

Important changes:

Versions of ACL2 preceding this one contain a subtle soundness bug! We found a flaw in our detection of subversive-recursions. The bug allowed some subversive recursions to slip through undetected.

We believe it would have been difficult to have exploited this flaw inadvertently. In particular, the following five conditions are necessary.

(1) Introduce a constrained function, say f, via an encapsulate.

(2) In the same encapsulation, define a clique of mutually recursive functions. This clique must be non-local and in :logic mode.

(3) In that mutually recursive clique, use the constrained function f (perhaps indirectly) so that the termination argument for the clique depends on properties of the witness for f. Thus, f or some other function dependent upon f, must be used in an argument in a recursive call or in a term governing a recursive call. Furthermore, the use of f must be such that the termination proof cannot be done without exploiting properties of the witness for f. Other uses of the constrained functions in the clique are ok.

(4) Fail to include the exploited properties of f among the constraints of the encapsulation.

(5) Later, outside the encapsulation, explicitly use a functional instantiation in which f is replaced by a function not enjoying the crucial properties.

See subversive-recursions for details.

Less important notes:

We have begun to write some introductory tutorial material for those who wish to learn to program in ACL2. Most of this material is HTML-based. See the Hyper-Card on the ACL2 home page. The documentation of verify-guards was improved to explain why one might wish to verify the ``guards'' of a defthm event. The missing documentation was noticed by John Cowles.

A bug was fixed in cross fertilization. The bug caused the system to report that it had substituted one term for another when in fact no substitution occurred. The bug was noticed by Bill McCune.


Acl2 Version 1.1 Notes
Major Section:  RELEASE-NOTES

The new features are extensively documented. The relevant topics are:

It is especially important to read all of of the documentation for books before trying to use books. However, the new :more keyword command is so handy for reading long documentation strings that we recommend you start with :doc more if reading at the terminal. Some documentation has been written for guards which you might find interesting.


Acl2 Version 1.2 Notes
Major Section:  RELEASE-NOTES

Hacker mode has been eliminated and programming mode has been added. Programming mode is unsound but does syntax checking and permits redefinitions of names. See :doc load-mode and :doc g-mode.

The arguments to ld have changed. Ld is now much more sophisticated. See ld.

For those occasions on which you wish to look at a large list structure that you are afraid to print, try (walkabout x state), where x is an Acl2 expression that evaluates to the structure in question. I am afraid there is no documentation yet, but it is similar in spirit to the Interlisp structure editor. You are standing on an object and commands move you around in it. E.g., 1 moves you to its first element, 2 to its second, etc.; 0 moves you up to its parent; nx and bk move you to its next sibling and previous sibling; pp prettyprints it; q exits returning nil; = exits returning the thing you're standing on; (= symb) assigns the thing you're standing on to the state global variable symb.

Several new hints have been implemented, including :by and :do-not. The old :do-not-generalize has been scrapped in favor of such new hints as :do-not (generalize elim). :By lets you say ``this goal is subsumed by'' a given lemma instance. The :by hint also lets you say ``this goal can't be proved yet but skip it and see how the rest of the proof goes.'' See hints.


Acl2 Version 1.3 Notes
Major Section:  RELEASE-NOTES

Programming mode has been eliminated. Instead, all functions have a ``color'' which indicates what can be done with the function. For example, :red functions can be executed but have no axioms describing them. Thus, :red functions can be introduced after passing a simple syntactic check and they can be redefined without undoing. But nothing of consequence can be proved about them. At the other extreme are :gold functions which can be executed and which also have passed both the termination and the guard verification proofs. The color of a function can be specified with the new xargs keyword, :color, which, if omitted defaults to the global setting of ld-color. Ld-color replaces load-mode. Setting ld-color to :red causes behavior similar to the old :g-mode. Setting ld-color to :gold causes behavior similar to the old :v-mode. It is possible to prototype your system in :red and then convert :red functions to :blue individually by calling verify-termination on them. They can then be converted to :gold with verify-guards. This allows us to undertake to verify the termination and guards of system functions. See :doc color for an introduction to the use of colors.

Type prescription rules have been added. Recall that in Nqthm, some rewrite rules were actually stored as ``type-prescriptions.'' Such rules allow the user to inform Nqthm's primitive type mechanism as to the kinds of shells returned by a function. Earlier versions of Acl2 did not have an analogous kind of rule because Acl2's type mechanism is complicated by guards. Version 1.3 supports type-prescription rules. See type-prescription.

Three more new rule-classes implement congruence-based rewriting. It is possible to identify a binary relation as an equivalence relation (see equivalence), to show that one equivalence relation refines another (see refinement) and to show that a given equivalence relation is maintained when rewriting a given function call, e.g., (fn ...xk...), by maintaining another equivalence relation while rewriting the kth argument (see congruence). If r has been shown to be an equivalence relation and then (implies hyps (r (foo x) (bar x))) is proved as a :rewrite rule, then instances of (foo x) will be replaced by corresponding instances of (bar x) provided the instance occurs in a slot where the maintainence of r-equivalence is known to be sufficient and hyps can be established as usual.

In Version 1.2, rule-classes were simple keywords, e.g., :rewrite or :elim. In Version 1.3, rule-classes have been elaborated to allow you to specify how the theorem ought to be used as a rule. That is, the new rule-classes allows you to separate the mathematical statement of the formula from its interpretation as a rule. See rule-classes.

Rules used to be named by symbols, e.g., car and car-cons were the names of rules. Unfortunately, this was ambiguous because there are three rules associated with function symbols: the symbolic definition, the executable counterpart, and the type-prescription; many different rules might be associated with theorems, depending on the rule classes. In Version 1.3 rules are named by ``runes'' (which is just short hand for ``rule names''). Example runes are (:definition car), (:executable-counterpart car), and (:type-prescription car . 1). Every rule added by an event has a different name and you can enable and disable them independently. See rune and see theories.

The identity function force, of one argument, has been added and given a special interpretation by the functions responsible for establishing hypotheses in backchaining: When the system fails to establish some hypothesis of the form (force term), it simply assumes it is true and goes on, delaying until later the establishment of term. In particular, pushes a new subgoal to prove term in the current context. When that subgoal is attacked, all of the resources of the theorem prover, not just rewriting, are brought to bear. Thus, for example, if you wish to prove the rule (implies (good-statep s) (equal (exec s n) s')) and it is your expectation that every time exec appears its first argument is a good-statep then you might write the rule as (implies (force (good-statep s)) (equal (exec s n) s')). This rule is essentially an unconditional rewrite of (exec s n) to s' that spawns the new goal (good-statep s). See force. Because you can now specify independently how a theorem is used as a rule, you need not write the force in the actual theorem proved. See rule-classes.

Version 1.3 supports a facility similar to Nqthm's break-lemma. See break-rewrite. You can install ``monitors'' on runes that will cause interactive breaks under certain conditions.

Acl2 also provides ``wormholes'' which allow you to write functions that cause interaction with the user but which do not require that you have access to state. See wormhole.

The rewriter now automatically backchains to stronger recognizers. There is no user hook to this feature but it may simplify some proofs with which older versions of Acl2 had trouble. For example, if the rewriter is trying to prove (rationalp (foo a b c)) it is now smart enough to try lemmas that match with (integerp (foo a b c)).


Acl2 Version 1.4 Notes
Major Section:  RELEASE-NOTES

Once again ld only takes one required argument, as the bind-flg has been deleted.

Three commands have been added in the spirit of :pe. :Pe! is similar to :pe but it prints all events with the given name, rather than just the most recent. The command :pf prints the corollary formula corresponding to a name or rune. The command :pl (print lemmas) prints rules whose top function symbol is the given name. See pe!, see pf, and see pl.

Book naming conventions have been changed somewhat. The once-required .lisp extension is now prohibited! Directories are supported, including a notion of ``connected book directory''. See book-name. Also, the second argument of certify-book is now optional, defaulting to 0.

Compilation is now supported inside the Acl2 loop. See comp and see set-compile-fns.

The default color is now part of the Acl2 world; see :doc default-color. Ld-color is no longer an ld special. Instead, colors are events; see the documentation for red, pink, blue, and gold.

A table exists for controlling whether Acl2 prints comments when it forces hypotheses of rules; see :doc force-table. Also, it is now possible to turn off the forcing of assumptions by disabling the definition of force; see force.

The event defconstant is no longer supported, but a very similar event, defconst, has been provided in its place. See defconst.

The event for defining congruence relations is now defcong (formerly, defcon).

Patterns are now allowed in :expand hints. See the documentation for :expand inside the documentation for hints.

We have improved the way we report rules used by the simplifier. All runes of the same type are reported together in the running commentary associated with each goal, so that for example, executable counterparts are listed separately from definitions, and rewrite rules are listed separately from linear rules. The preprocessor now mentions ``simple'' rules; see simple.

The mechanism for printing warning messages for new rewrite rules, related to subsumption, now avoids worrying about nonrecursive function symbols when those symbols are disabled. These messages have also been eliminated for the case where the old rule is a :definition rule.

Backquote has been modified so that it can usually provide predictable results when used on the left side of a rewrite rule.

Time statistics are now printed even when an event fails.

The Acl2 trace package has been modified so that it prints using the values of the Lisp globals *print-level* and *print-length* (respectively).

Table has been modified so that the :clear option lets you replace the entire table with one that satisfies the val and key guards (if any); see table.

We have relaxed the translation rules for :measure hints to defun, so that the the same rules apply to these terms that apply to terms in defthm events. In particular, in :measure hints mv is treated just like list, and state receives no special handling.

The loop-stopper test has been relaxed. The old test required that every new argument be strictly less than the corresponding old argument in a certain term-order. The new test uses a lexicographic order on term lists instead. For example, consider the following rewrite rule.

   (variable-update var1
                    val1 (variable-update var2 val2 vs))
   (variable-update var2
                    val2 (variable-update var1 val1 vs)))
This rule is permutative. Now imagine that we want to apply this rule to the term
  (variable-update u y (variable-update u x vs)).
Since the actual corresponding to both var1 and var2 is u, which is not strictly less than itself in the term-order, this rule would fail to be applied in this situation when using the old test. However, since the pair (u x) is lexicographically less than the pair (u y) with respect to our term-order, the rule is in fact applied using our new test.

Messages about events now contain a space after certain left parentheses, in order to assist emacs users. For example, the event

  (defthm abc (equal (+ (len x) 0) (len x)))
leads to a summary containing the line
  Form:  ( DEFTHM ABC ...)
and hence, if you search backwards for ``(defthm abc'', you won't stop at this message.

More tautology checking is done during a proof; in fact, no goal printed to the screen, except for the results of applying :use and :by hints or the top-level goals from an induction proof, are known to Acl2 to be tautologies.

The ld-query-control-alist may now be used to suppress printing of queries; see ld-query-control-alist.

Warning messages are printed with short summary strings, for example the string ``Use'' in the following message.

  Acl2 Warning [Use] in DEFTHM:  It is unusual to :USE an enabled
  :REWRITE or :DEFINITION rule, so you may want to consider
  disabling FOO.
At the end of the event, just before the time is printed, all such summary strings are printed out.

The keyword command :u has been introduced as an abbreviation for :ubt :max. Printing of query messages is suppressed by :u.

The keyword :cheat is no longer supported by any event form.

Some irrelevant formals are detected; see irrelevant-formals.

A bug in the application of metafunctions was fixed: now if the output of a metafunction is equal to its input, the application of the metafunction is deemed unsuccessful and the next metafunction is tried.

An example has been added to the documentation for equivalence to suggest how to make use of equivalence relations in rewriting.

The following Common Lisp functions have been added to Acl2: alpha-char-p, upper-case-p, lower-case-p, char-upcase, char-downcase, string-downcase, string-upcase, and digit-charp-p.

A documentation section called proof-checker has been added for the interactive facility, whose documentation has been slightly improved. See in particular the documentation for proof-checker, verify, and macro-command.

A number of events that had been inadvertently disallowed in books are now permitted in books. These are: defcong, defcor, defequiv, defrefinement, defstub, and verify-termination.


Acl2 Version 1.5 Notes
Major Section:  RELEASE-NOTES

Acl2 now allows ``complex rationals,'' which are complex numbers whose real parts are rationals and whose imaginary parts are non-zero rationals. See complex.

A new way of handling forced hypotheses has been implemented. Rather than cause a case split at the time the force occurs, we complete the main proof and then embark on one or more ``forcing rounds'' in which we try to prove the forced hypotheses. See forcing-round. To allow us to compare the new handling of force with the old, Version 1.5 implements both and uses a flag in state to determine which method should be used. Do (assign old-style-forcing t) if you want force to be handled as it was in Version 1.4. However, we expect to eliminate the old-style forcing eventually because we think the new style is more effective. To see the difference between the two approaches to forcing, try proving the associativity of append under both settings of old-style-forcing. To get the new behavior invoke:

(thm (implies (and (true-listp a) (true-listp b))
              (equal (append (append a b) c)
                     (append a (append b c)))))
Then (assign old-style-forcing t) and invoke the thm command above again.

A new :cases hints allows proof by cases. See hints.

Include-book and encapsulate now restore the acl2-defaults-table when they complete. See include-book and see encapsulate.

The guards on many Acl2 primitives defined in axioms.lisp have been weakened to permit them to be used in accordance with lisp custom and tradition.

It is possible to attach heuristic filters to :rewrite rules to limit their applicability. See syntaxp.

A tutorial has been added; see acl2-tutorial.

Events now print the Summary paragraph listing runes used, time, etc., whether they succeed or fail. The format of the ``failure banner'' has been changed but still has multiple asterisks in it. Thm also prints a Summary, whether it succeeds or fails; but thm is not an event.

A new event form skip-proofs has been added; see skip-proofs.

A user-specific customization facility has been added in the form of a book that is automatically included, if it exists on the current directory. See acl2-customization.

A facility for conditional metalemmas has been implemented; see meta.

The acceptable values for ld-skip-proofsp have changed. In the old version (Version 1.4), a value of t meant that proofs and local events are to be skipped. In Version 1.5, a value of t means proofs (but not local events) are to be skipped. A value of 'include-book means proofs and local events are to be skipped. There are two other, more obscure, acceptable values. See ld-skip-proofsp.

In order to turn off the forcing of assumptions, one should now disable the :executable-counterpart of force (rather than the :definition of force, as in the previous release); see force.

The macros enable-forcing and disable-forcing make it convenient to enable or disable forcing. See enable-forcing and see disable-forcing.

The new commands :pr and :pr! print the rules created by an event or command. See pr and see pr!.

The new history commands :puff and :puff* will replace a compound command such as an encapsulate or include-book by the sequence of events in it. That is, they ``puff up'' or ``lift'' the subevents of a command to the command level, eliminating the formerly superior command and lengthening the history. This is useful if you want to ``partially undo'' an encapsulate or book or other compound command so you can experiment. See puff and see puff*.

Theory expressions now are allowed to use the free variable world and prohibited from using the free variable state. See theories, although it is essentially the same as before except it mentions world instead of state. See world for a discussion of the Acl2 logical world. Allowing in-theory events to be state-sensitive violated an important invariant about how books behaved.

Table keys and values now are allowed to use the free variable world and prohibited from using the free variable state. See the note above about theory expressions for some explanation.

The macro for minus, -, used to expand (- x 3) to (+ x -3) and now expands it to (+ -3 x) instead. The old macro, if used in the left-hand sides of rewrite rules, produced inapplicable rules because the constant occurs in the second argument of the +, but potential target terms generally had the constant in the first argument position because of the effect of commutativity-of-+.

A new class of rule, :linear-alias rules, allows one to implement the nqthm package and similar hacks in which a disabled function is to be known equivalent to an arithmetic function. See linear-alias.

A new class of rule, :built-in-clause rules, allows one to extend the set of clauses proved silently by defun during measure and guard processing. See built-in-clauses.

The new command pcb! is like pcb but sketches the command and then prints its subsidiary events in full. See pcb!.

:Rewrite class rules may now specify the :loop-stopper field. See rule-classes and see loop-stopper.

The rules for how loop-stoppers control permutative rewrite rules have been changed. One effect of this change is that now when the built-in commutativity rules for + are used, the terms a and (- a) are permuted into adjacency. For example, (+ a b (- a)) is now normalized by the commutativity rules to (+ a (- a) b); in Version 1.4, b was considered syntactically smaller than (- a) and so (+ a b (- a)) is considered to be in normal form. Now it is possible to arrange for unary functions be be considered ``invisible'' when they are used in certain contexts. By default, unary-- is considered invisible when its application appears in the argument list of binary-+. See loop-stopper and see set-invisible-fns-alist.

Extensive documentation has been provided on the topic of Acl2's ``term ordering.'' See term-order.

Calls of ld now default ld-error-action to :return rather than to the current setting.

The command descriptor :x has been introduced and is synonymous with :max, the most recently executed command. History commands such as :pbt print a :x beside the most recent command, simply to indicate that it is the most recent one.

The command descriptor :x-23 is synonymous with (:x -23). More generally, every symbol in the keyword package whose first character is #\x and whose remaining characters parse as a negative integer is appropriately understood. This allows :pbt :x-10 where :pbt (:max -10) or :pbt (:here -10) were previously used. The old forms are still legal.

The order of the arguments to defcong has been changed.

The simplifier now reports the use of unspecified built-in type information about the primitives with the phrase ``primitive type reasoning.'' This phrase may sometimes occur in situations where ``propositional calculus'' was formerly credited with the proof.

The function pairlis has been replaced in the code by a new function pairlis$, because Common Lisp does not adequately specify its pairlis function.

Some new Common Lisp functions have been added, including logtest, logcount, integer-length, make-list, remove-duplicates, string, and concatenate. The source file /slocal/src/acl2/axioms.lisp is the ultimate reference regarding Common Lisp functions in Acl2.

The functions defuns and theory-invariant have been documented. See defuns and see theory-invariant.

A few symbols have been added to the list *acl2-exports*.

A new key has been implemented for the acl2-defaults-table, :irrelevant-formals-ok. See set-irrelevant-formals-ok.

The connected book directory, cbd, must be nonempty and begin and end with a slash. It is set (and displayed) automatically upon your first entry to lp. You may change the setting with set-cbd. See cbd.

:oops will undo the last :ubt. See oops.

Documentation has been written about the ordinals. See e0-ordinalp and see e0-ord-<.

The color events -- (red), (pink), (blue), and (gold) -- may no longer be enclosed inside calls of local, for soundness reasons. In fact, neither may any event that sets the acl2-defaults-table. See embedded-event-form.

See ld-keyword-aliases for an example of how to change the exit keyword from :q to something else.

The attempt to install a monitor on :rewrite rules stored as simple abbreviations now causes an error because the application of abbreviations is not tracked.

A new message is sometimes printed by the theorem prover, indicating that a given simplification is ``specious'' because the subgoals it produces include the input goal. In Version 1.4 this was detected but not reported, causing behavior some users found bizarre. See specious-simplification.

:Definition rules are no longer always required to specify the :clique and :controller-alist fields; those fields can be defaulted to system-determined values in many common instances. See definition.

A warning is printed if a macro form with keyword arguments is given duplicate keyword values. Execute (thm t :doc nil :doc "ignored") and read the warning printed.

A new restriction has been placed on encapsulate. Non-local recursive definitions inside the encapsulate may not use, in their tests and recursive calls, the constrained functions introduced by the encapsulate. See subversive-recursions. (Note added in Version 2.3: Subversive recursions were first recognized by us here in Version 1.5, but our code for recognizing them was faulty and the bug was not fixed until Version 2.3.)

The events defequiv, defcong, defrefinement, and defevaluator have been reimplemented so that they are just macros that expand into appropriate defthm or encapsulate events; they are no longer primitive events. See the documentation of each affected event.

The defcor event, which was a shorthand for a defthm that established a corollary of a named, previously proved event, has been eliminated because its implementation relied on a technique we have decided to ban from our code. If you want the effect of a defcor in Version 1.5 you must submit the corresponding defthm with a :by hint naming the previously proved event.

Error reporting has been improved for inappropriate in-theory hints and events, and for syntax errors in rule classes, and for non-existent filename arguments to ld.

Technical Note: We now maintain the Third Invariant on type-alists, as described in the Essay on the Invariants on Type-alists, and Canonicality. This change will affect some proofs, for example, by causing a to rewrite more quickly to c when (equiv a b) and (equiv b c) are both known and c is the canonical representative of the three.