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# 1 The ACE Package

### Sections

1. Using ACE as a Default for Coset Enumerations
2. Using ACE Directly to Generate a Coset Table
3. Using ACE Directly to Test whether a Coset Enumeration Terminates
4. Writing ACE Standalone Input Files to Generate a Coset Table
5. Using ACE Interactively
6. Accessing ACE Examples with ACEExample and ACEReadResearchExample
7. General Warnings regarding the Use of Options
8. The ACEData Record
9. Setting the Verbosity of ACE via Info and InfoACE
10. Acknowledgements
11. Changes from earlier versions

The ``Advanced Coset Enumerator'' ACE:

    ACE coset enumerator (C) 1995-2001 by George Havas and Colin Ramsay

http://www.itee.uq.edu.au/~cram/ce.html

can be called from within GAP through an interface, written by Alexander Hulpke and Greg Gamble, which is described in this manual.

The interface links to an external binary and therefore is only usable under UNIX (see Section Installing the ACE Package for how to install ACE). It will not work on Windows or a pre-MacOSX Macintosh. The current version requires at least GAP 4.4. Users who still have GAP 4.3, must use ACE 4.1. (The first versions supported GAP 4.2.) ACE can be used through this interface in a variety of ways:

- one may supplant the usual GAP coset enumerator (see Section Using ACE as a Default for Coset Enumerations),

- one may generate a coset table using ACE without redefining the usual GAP coset enumerator (see Section Using ACE Directly to Generate a Coset Table),

- one may simply test whether a coset enumeration will terminate (see Section Using ACE Directly to Test whether a Coset Enumeration Terminates),

- one may use GAP to write a script for the ACE standalone (see Section Writing ACE Standalone Input Files to Generate a Coset Table), and

- one may interact with the ACE standalone from within GAP (see Section Using ACE Interactively). Among other things, the interactive ACE interface functions (described in Chapter Functions for Using ACE Interactively) enable the user to search for subgroups of a group (see the note of Section Using ACE Interactively).

Each of these ways gives the user access to a welter of options, which are discussed in full in Chapters Options for ACE and Strategy Options for ACE. Yet more options are provided in Appendix Other ACE Options, but please take note of the Appendix's introductory paragraph. Check out Appendix Examples for numerous examples of the ACE commands.

Note: Some care needs to be taken with options; be sure to read Section General Warnings regarding the Use of Options and the introductory sections of Chapter Options for ACE for some warnings regarding them and a general discussion of their use, before using any of the functions provided by this interface to the ACE binary.

## 1.1 Using ACE as a Default for Coset Enumerations

After loading ACE (see Section Loading the ACE Package), if you want to use the ACE coset enumerator as a default for all coset enumerations done by GAP (which may also get called indirectly), you can achieve this by setting the global variable `TCENUM` to `ACETCENUM`.

    gap> TCENUM:=ACETCENUM;;

This sets the function `CosetTableFromGensAndRels` (see Section Coset Tables and Coset Enumeration in the GAP Reference Manual) to be the function `ACECosetTableFromGensAndRels` (described in Section Using ACE Directly to Generate a Coset Table), which then can be called with all the options defined for the ACE interface, not just the options `max` and `silent`. If `TCENUM` is set to `ACETCENUM` without any further action, the `default` strategy (option) of the ACE enumerator will be used (see Chapter Strategy Options for ACE).

You can switch back to the coset enumerator built into the GAP library by assigning `TCENUM` to `GAPTCENUM`.

    gap> TCENUM:=GAPTCENUM;;

## 1.2 Using ACE Directly to Generate a Coset Table

If, on the other hand you do not want to set up ACE globally for your coset enumerations, you may call the ACE interface directly, which will allow you to decide for yourself, for each such call, which options you want to use for running ACE. Please note the warnings regarding options in Section General Warnings regarding the Use of Options. The functions discussed in this and the following section (`ACECosetTableFromGensAndRels` and `ACEStats`) are non-interactive, i.e. by their use, a file with your input data in ACE readable format will be handed to ACE and you will get the answer back in GAP format. At that moment however the ACE job is terminated, that is, you cannot send any further questions or requests about the result of that job to ACE. For an interactive use of ACE from GAP see Section Using ACE Interactively and Chapter Functions for Using ACE Interactively.

Using the ACE interface directly to generate a coset table is done by either of

- `ACECosetTableFromGensAndRels(` fgens`,` rels`,` sgens`[:` options`] ) F`
- `ACECosetTable(` fgens`,` rels`,` sgens`[:` options`] ) F`

  Here fgens is a list of free generators, rels a list of words in these generators giving relators for a finitely presented group, and sgens the list of subgroup generators, again expressed as words in the free generators. All these are given in the standard GAP format (see Chapter Finitely Presented Groups of the GAP Reference Manual). Note that the 3-argument form of `ACECosetTable` described here is merely a synonym for `ACECosetTableFromGensAndRels`, and that `ACECosetTable` may be called in a different way in an interactive ACE session (see Sections Using ACE Interactively and ACECosetTable!interactive).

  Behind the colon any selection of the options available for the interface (see Chapters Options for ACE and Strategy Options for ACE) can be given, separated by commas like record components. These can be used e.g. to preset limits of space and time to be used, to modify input and output and to modify the enumeration procedure. Note that strategies are simply special options that set a number of the options, detailed in Chapter Options for ACE, all at once.

  Please see Section General Warnings regarding the Use of Options for a discussion regarding global and local passing of options, and the non-orthogonal nature of ACE's options.

  Each of `ACECosetTableFromGensAndRels` and `ACECosetTable` calls the ACE binary and, if successful, returns a standard coset table, as a GAP list of lists. At the time of writing, two coset table standardisations schemes were possible: `lenlex` and `semilenlex` (see Section Coset Table Standardisation Schemes). The user may control which standardisation scheme is used by selecting either the `lenlex` (see option lenlex) or `semilenlex` (see option semilenlex) option; otherwise (since GAP 4.3) the table is standardised according to GAP's the value of `CosetTableStandard` (which by default is `lenlex`; for GAP 4.2, the variable `CosetTableStandard` didn't exist and the default standardisation scheme was `semilenlex`). We provide `IsACEStandardCosetTable` (see IsACEStandardCosetTable) to determine whether a table (list of lists) is standard relative to GAP's default standardisation scheme, or with the use of options (e.g. `lenlex` or `semilenlex`) to another standardisation scheme.

  If the determination of a coset table is unsuccessful, then one of the following occurs:

  - with the `incomplete` option (see option incomplete) an incomplete coset table is returned (as a list of lists), with zeros in positions where valid coset numbers could not be determined; or

  - with the `silent` option (see option silent), `fail` is returned; or

  - a `break`-loop is entered. This last possibility is discussed in detail via the example that follows.

  The example given below is the call for a presentation of the Fibonacci group F(2,7) for which we shall discuss the impact of various options in Section Fun with ACEExample. Observe that in the example, no options are passed, which means that ACE uses the `default` strategy (see Chapter Strategy Options for ACE).

      gap> F:= FreeGroup( "a", "b", "c", "d", "e", "x", "y");;
      gap> a:= F.1;; b:= F.2;; c:= F.3;; d:= F.4;; e:= F.5;; x:= F.6;; y:= F.7;;
      gap> fgens:= [a, b, c, d, e, x, y,];;
      gap> rels:= [ a*b*c^-1, b*c*d^-1, c*d*e^-1, d*e*x^-1,
      >             e*x*y^-1, x*y*a^-1, y*a*b^-1];;
      gap> ACECosetTable(fgens, rels, [c]);;

  In computing the coset table, `ACECosetTableFromGensAndRels` must first do a coset enumeration (which is where ACE comes in!). If the coset enumeration does not finish in the preset limits a `break`-loop is entered, unless the `incomplete` (see option incomplete) or `silent` (see option silent) options is set. In the event that a `break`-loop is entered, don't despair or be frightened by the word `Error`; by tweaking ACE's options via the `SetACEOptions` function that becomes available in the `break`-loop and then typing `return;` it may be possible to help ACE complete the coset enumeration (and hence successfully compute the coset table); if not, you will end up in the `break`-loop again, and you can have another go (or `quit;` if you've had enough). The `SetACEOptions` function is a no-argument function; it's there purely to pass options (which, of course, are listed behind a colon (`:`) with record components syntax). Let's continue the Fibonacci example above, redoing the last command but with the option `max := 2` (see option max), so that the coset enumeration has only two coset numbers to play with and hence is bound to fail to complete, putting us in a `break`-loop.

      gap> ACECosetTable(fgens, rels, [c] : max := 2);
      Error, no coset table ...
       the `ACE' coset enumeration failed with the result:
       OVERFLOW (a=2 r=1 h=1 n=3; l=5 c=0.00; m=2 t=2)
       called from
      <function>( <arguments> ) called from read-eval-loop
      Entering break read-eval-print loop ...
       try relaxing any restrictive options
       e.g. try the `hard' strategy or increasing `workspace'
       type: '?strategy options' for info on strategies
       type: '?options for ACE' for info on options
       type: 'DisplayACEOptions();' to see current ACE options;
       type: 'SetACEOptions(:<option1> := <value1>, ...);'
       to set <option1> to <value1> etc.
       (i.e. pass options after the ':' in the usual way)
       ... and then, type: 'return;' to continue.
       Otherwise, type: 'quit;' to quit to outer loop.
      brk> SetACEOptions(: max := 0);
      brk> return;
      [ [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ],
        [ 1 ], [ 1 ], [ 1 ], [ 1 ] ]

  Observe how the lines after the ```Entering break read-eval-print loop`'' announcement tell you exactly what to do (for GAP 4.2 these lines are instead `Info`-ed before the `break`-loop announcement). At the `break`-loop prompt `brk>` we relaxed all restrictions on `max` (by re-setting it to 0) and typed `return;` to leave the `break`-loop. The coset enumeration was then successful, allowing the computation of what turned out to be a trivial coset table. Despite the fact that the eventual coset table only has one line (i.e. there is exactly one coset number) ACE did need to define more than 2 coset numbers. To find out just how many were required before the final collapse, let's set the `InfoLevel` of `InfoACE` (see SetInfoACELevel) to 2, so that the ACE enumeration result is printed.

      gap> SetInfoACELevel(2);
      gap> ACECosetTable(fgens, rels, [c]);
      #I  INDEX = 1 (a=1 r=2 h=2 n=2; l=6 c=0.01; m=2049 t=3127)
      [ [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ],
        [ 1 ], [ 1 ], [ 1 ], [ 1 ] ]

  The enumeration result line is the `Info` line beginning ```#I` ''. Appendix The Meanings of ACE's Output Messages explains how to interpret such output messages from ACE. In particular, it explains that `t=3127` tells us that a `t`otal number of 3127 coset numbers needed to be defined before the final collapse to 1 coset number. Using some of the many options that ACE provides, one may achieve this result more efficiently, e.g. with the `purec` strategy (see option purec):

      gap> ACECosetTable(fgens, rels, [c] : purec);
      #I  INDEX = 1 (a=1 r=2 h=2 n=2; l=4 c=0.00; m=332 t=332)
      [ [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ], [ 1 ],
        [ 1 ], [ 1 ], [ 1 ], [ 1 ] ]

  ACE needs to define a `t`otal number of only (relatively-speaking) 332 coset numbers before the final collapse to 1 coset number.

  Notes: To initiate the coset enumeration, the `start` option (see option start) is quietly inserted after the user's supplied options, unless the user herself supplies one of the enumeration-invoking options, which are: `start`, or one of its synonyms, `aep` (see option aep) or `rep` (see option rep).

  When a user calls `ACECosetTable` with the `lenlex` option (see option lenlex), occasionally it is necessary to enforce `asis`=1 (see option asis), which may be counter to the desires of the user. The occasions where this action is necessary are precisely those for which, for the arguments gens and rels of `ACECosetTable`, `IsACEGeneratorsInPreferredOrder` would return `false`.

  The non-interactive `ACECosetTable` and `ACECosetTableFromGensAndRels` now use an iostream to communicate with the ACE binary in order to avoid filling up a temporary directory with an incomplete coset table in the case where an enumeration overflows. This is generally advantageous. However, on some systems, it may not be advisable to repeatedly call `ACECosetTable` or `ACECosetTableFromGensAndRels` (e.g. in a loop), since one may run out of the pseudo ttys used for iostreams. If you encounter this problem, consider using an adaptation of the usage of the interactive forms of `ACECosetTable` and `ACEStats` (see ACECosetTable!interactive and ACEStats!interactive), together with `ACEStart` initialisation steps, that is sketched in the schema below. For the following code, it is imagined the scenario is one of looping over several possibilities of fgens, rels and sgens; the two special forms of `ACEStart` used, allow one to continually re-use a single interactive ACE process (i.e. only one iostream is used).

  `# start the interactive ACE process with no group information`
  `procId := ACEStart(0);`
  `while` expr `do`
  `fgens := ...; rels := ...; sgens := ...;`
  `ACEStart(procId, fgens, rels, sgens :` options`);`
  `if ACEStats(procId).index > 0 then`
  `table := ACECosetTable(procId);`
  `...`
  `fi;`
  `od;`

  For an already calculated coset table, we provide the following function to determine whether or not it has been standardised.

- `IsACEStandardCosetTable(` table`[:` option`] ) F`

  returns `true` if table (a list of lists of integers) is standard according to GAP's default or the option (either `lenlex` or `semilenlex`) standardisation scheme, or `false` otherwise. See Section Coset Table Standardisation Schemes for a detailed discussion of the `lenlex` and `semilenlex` standardisation schemes.

  Note: Essentially, `IsACEStandardCosetTable` extends the GAP function `IsStandardized`.

  Users who wish their coset tables to use `ACECosetTable` with the `lenlex` (see option lenlex) option, which causes `lenlex` standardisation to occur at the ACE (rather than GAP) level, should be aquainted with the following function.

- `IsACEGeneratorsInPreferredOrder(` gens`,` rels`) F`

  returns `true` if gens, a list of free group generators, are in an order which will not be changed by ACE, for the group with presentation á gens | rels ñ, where rels is a list of relators (i.e. words in the generators gens). `IsACEGeneratorsInPreferredOrder` may also be called in a different way for interactive ACE processes (see IsACEGeneratorsInPreferredOrder!interactive).

  For a presentation with more than one generator, the first generator of which is an involution and the second is not, ACE prefers to switch the first two generators. `IsACEGeneratorsInPreferredOrder` returns `true` if the order of the generators gens would not be changed within ACE and `false`, otherwise. (Technically, by ``involution'' above, we really mean ``a generator `x` for which there is a relator in rels of form `x*x` or `x^2`''. Such a generator may, of course, turn out to actually be the identity.)

  Guru Notes: If `IsACEGeneratorsInPreferredOrder(`gens`,` rels`)` would return `false`, it is possible to enforce a user's order of the generators within ACE, by setting the option `asis` (see option asis) to 1 and, by passing the relator that determines that ``gens`[1]` (which we will assume is `x`) has order at most 2, as: `x*x` (rather than `x^2`). Behind the scenes this is precisely what is done, if necessary, when `ACECosetTable` is called with the `lenlex` option.

  The user may avoid all the technicalities by either not using the `lenlex` option (and allowing GAP to take care of the `lenlex` standardisation), or by swapping the first two generators in those cases where `IsACEGeneratorsInPreferredOrder(`gens`,` rels`)` would return `false`.

  ## 1.3 Using ACE Directly to Test whether a Coset Enumeration Terminates

  If you only want to test, whether a coset enumeration terminates, and don't want to transfer the whole coset table to GAP, you can call

- `ACEStats(` fgens`,` rels`,` sgens`[:` options`] ) F`

  which calls ACE non-interactively to do the coset enumeration, the result of which is parsed and returned as a GAP record with fields

  `index`
    the index of the subgroup á sgens ñ in á fgens | rels ñ, or 0 if the enumeration does not succeed;

  `cputime`
    the total CPU time used as an integer number of `cputimeUnits` (the next field);

  `cputimeUnits`
    the units of the `cputime` field, e.g. `"10^-2 seconds"`;

  `activecosets`
    the number of currently active (i.e. alive) coset numbers (see Section Coset Statistics Terminology);

  `maxcosets`
    the maximum number of alive coset numbers at any one time in the enumeration (see Section Coset Statistics Terminology); and

  `totcosets`
    the total number of coset numbers that were defined in the enumeration (see Section Coset Statistics Terminology).

  Options (see Chapters Options for ACE and Strategy Options for ACE) are used in exactly the same way as for `ACECosetTableFromGensAndRels`, discussed in the previous section; and the same warnings alluded to previously, regarding options (see Section General Warnings regarding the Use of Options), apply.

  Notes: To initiate the coset enumeration, the `start` option (see option start) is quietly inserted after the user's supplied options, unless the user herself supplies one of the enumeration-invoking options, which are: `start`, or one of its synonyms, `aep` (see option aep) or `rep` (see option rep).

  The fields of the `ACEStats` record are determined by parsing a ``results message'' (see Appendix The Meanings of ACE's Output Messages) from ACE.

  `ACEStats` may also be called in a different way in an interactive ACE session (see ACEStats!interactive).

  ## 1.4 Writing ACE Standalone Input Files to Generate a Coset Table

  If you want to use ACE as a standalone with its own syntax, you can write an ACE standalone input file by calling `ACECosetTable` with three arguments (see ACECosetTableFromGensAndRels) and the option `aceinfile :=` filename`` (see option aceinfile). This will keep the input file for the ACE standalone produced by the GAP interface under the file name filename (and just return) so that you can perform interactive work in the standalone.

  ## 1.5 Using ACE Interactively

  An interactive ACE process is initiated with the command

- `ACEStart(` fgens`,` rels`,` sgens`[:`options`] ) F`

  whose arguments and options are exactly as for `ACECosetTableFromGensAndRels` and `ACEStats`, as discussed in Sections Using ACE Directly to Generate a Coset Table and Using ACE Directly to Test whether a Coset Enumeration Terminates. The usual warnings regarding options apply (see Section General Warnings regarding the Use of Options). `ACEStart` has a number of other forms (see ACEStart!details).

  The return value is an integer (numbering from 1) which represents the running process. (It is possible to have more than one interactive process running at once.) The integer returned may be used to index which of these processes an interactive ACE function should be applied to.

  An interactive ACE process is terminated with the command

- `ACEQuit(` i`) F`

  where i is the integer returned by `ACEStart` when the process was begun. `ACEQuit` may also be called with no arguments (see ACEQuit!details).

  We discuss each of these commands, as well as the range of functions which enable one to access features of the ACE standalone not available non-interactively, in depth, in Chapter Functions for Using ACE interactively.

  Note:

  ACE not only allows one to do a coset enumeration of a group by a given (and then fixed) subgroup but it also allows one to search for subgroups by starting from a given one (possibly the trivial subgroup) and then augmenting it by adding new subgroup generators either explicitly via `ACEAddSubgroupGenerators` (see ACEAddSubgroupGenerators) or implicitly by introducing coincidences (see `ACECosetCoincidence`: ACECosetCoincidence, or `ACERandomCoincidences`: ACERandomCoincidences); or one can find smaller subgroups by deleting subgroup generators via `ACEDeleteSubgroupGenerators` (see ACEDeleteSubgroupGenerators).

  ## 1.6 Accessing ACE Examples with ACEExample and ACEReadResearchExample

  There are a number of examples available in the `examples` directory, which may be accessed via

- `ACEExample() F`
- `ACEExample(` examplename`[:`options`] ) F`
- `ACEExample(` examplename`,` ACEfunc`[:`options`] ) F`

  where examplename is a string, the name of an example (and corresponding file in the `examples` directory); and ACEfunc is the ACE function with which the example is to be executed.

  If `ACEExample` is called with no arguments, or with the argument: `"index"` (meant in the sense of ``list''), or with a non-existent example name, a list of available examples is displayed. See Section Fun with ACEExample where the list is displayed.

  By default, examples are executed via `ACEStats`. However, if `ACEExample` is called with a second argument (choose from the (other) alternatives: `ACECosetTableFromGensAndRels` (or, equivalently `ACECosetTable`), or `ACEStart`), the example is executed using that function, instead. Note that, whereas the first argument appears in double quotes (since it's a string), the second argument does not (since it's a function); e.g. to execute example `"A5"` with function `ACECosetTable`, one would type: `ACEExample("A5", ACECosetTable);`.

  `ACEExample` also accepts user options, which may be passed either globally (i.e. by using `PushOptions` to push them onto the `OptionsStack`) or locally behind a colon after the `ACEExample` arguments, and they are passed to `ACEStats` or ACEfunc as if they were appended to the existing options of examplename; in this way, the user may over-ride any or all of the options of examplename. This is done by passing an option `aceexampleoptions` (see option aceexampleoptions), which sets up a mechanism to reverse the usual order in which options of recursively called functions are pushed onto the `OptionsStack`. The option `aceexampleoptions` is not a user option; it is intended only for internal use by `ACEExample`, for the above purpose. In the portion of the output due to the `echo` option, if one has passed options to `ACEExample`, one will see `aceexampleoptions` listed first and the result of the interaction of examplename's options and the additional options.

  Consider the example `"A5"`. The effect of running

      gap> ACEExample("A5", ACEStart);

  is essentially equivalent to executing:

      gap> file := Filename(DirectoriesPackageLibrary("ace", "examples"), "A5");;
      gap> ACEfunc := ACEStart;;
      gap> ReadAsFunction(file)();

  except that some internal ``magic'' of `ACEExample` edits the example file and displays equivalent commands a user ``would'' execute. If the user has passed options to `ACEExample` these appear in a ```# User Options`'' block after the original options of the example in the `Info` portion of the output. By comparing with the portion of the output from the `echo` option (unless the user has over-ridden the `echo` option), the user may directly observe any over-riding effects of user-passed options.

  Please see Section Fun with ACEExample for some sample interactions with `ACEExample`.

  Notes

  Most examples use the `mess` ( = `messages`) option. To see the effect of this, it is recommended to do: `SetInfoACELevel(3);` before calling `ACEExample`, with an example.

  The coset table output from `ACEExample`, when called with many of the examples and with the ACE function `ACECosetTableFromGensAndRels` is often quite long. Recall that the output may be suppressed by following the (`ACEExample`) command with a double semicolon (`;;`).

  Also, try `SetInfoACELevel(2);` before calling `ACEExample`, with an example.

  If you unexpectedly observe `aceexampleoptions` in your output, then most probably you have unintentionally passed options by the global method, by having a non-empty `OptionsStack`. One possible remedy is to use `FlushOptionsStack();` (see FlushOptionsStack), before trying your `ACEExample` call again.

  As discussed in Section Interpretation of ACE Options, there is generally no sensible meaning that can be attributed to setting a strategy option (see Chapter Strategy Options for ACE) to `false`; if you wish to nullify the effect of a strategy option, pass another strategy option, e.g. pass the `default` (see option default) strategy option.

  Also provided are:

- `ACEReadResearchExample(` filename`) F`
- `ACEReadResearchExample() F`

  which perform `Read` (see Section Read in the GAP Reference Manual) on filename or, with no argument, the file with filename `"pgrelfind.g"` in the `res-examples` directory; e.g. the effect of running

      gap> ACEReadResearchExample("pgrelfind.g");

  is equivalent to executing:

      gap> Read( Filename(DirectoriesPackageLibrary("ace", "res-examples"),
      >                   "pgrelfind.g") );

  The examples provided in the `res-examples` directory were used to solve a genuine research problem, the results of which are reported in CHHR01. Please see Section Using ACEReadResearchExample for a detailed description and examples of its use.

- `ACEPrintResearchExample(` example-filename`) F`
- `ACEPrintResearchExample(` example-filename`,` output-filename`) F`

  print the ``essential'' contents of the file example-filename in the `res-examples` directory to the terminal, or with two arguments to the file output-filename; example-filename and output-filename should be strings. `ACEPrintResearchExample` is provided to make it easy for users to copy and edit the examples for their own purposes.

  ## 1.7 General Warnings regarding the Use of Options

  Firstly, let us mention (and we will remind you later) that an ACE strategy is merely a special option of ACE that sets a number of the options described in Chapter Options for ACE all at once. The strategy options are discussed in their own chapter (Chapter Strategy Options for ACE).

  In Section Passing ACE Options, we describe the two means provided by GAP by which ACE options may be passed. In Section Warnings regarding Options, we discuss how options ``left over'' from previous calculations can upset subsequent calculations; and hence, to ``clear the decks'' in such circumstances, why we have provided `FlushOptionsStack` (see FlushOptionsStack). However, removing `OptionsStack` options does not remove the options previously passed to an interactive ACE process; Section Warnings regarding Options discusses that too.

  Note, that the ACE package treatment of options is an ``enhancement'' over the general way GAP treats options. Firstly, the ACE binary allows abbreviations and mixed case of options and so the ACE package does also, as much as is possible (see Abbreviations and mixed case for ACE Options). Secondly, since ACE's options are in general not orthogonal, the order in which they are put to ACE is, in general, honoured (see Honouring of the order in which ACE Options are passed). Thirdly, the ACE binary's concept of a boolean option is slightly different to that of GAP's; Section Interpretation of ACE Options discusses, in particular, how an option detected by GAP as `true` is passed to the ACE binary.

  Finally, Section What happens if no ACE Strategy Option or if no ACE Option is passed discusses what happens if no option is selected.

  ## 1.8 The ACEData Record

- `ACEData V`

  is a GAP record in which the essential data for an ACE session within GAP is stored; its fields are:

  `binary`
    the path of the ACE binary;

  `tmpdir`
    the path of the temporary directory containing the non-interactive ACE input and output files (also see ACEDirectoryTemporary below);

  `ni`
    the data record for a non-interactive ACE process;

  `io`
    list of data records for `ACEStart` (see below and ACEStart) processes;

  `infile`
    the full path of the ACE input file;

  `outfile`
    the full path of the ACE output file; and

  `version`
    the version of the current ACE binary. More detailed information regarding the compilation of the binary is given by `ACEBinaryVersion` (see ACEBinaryVersion).

  Non-interactive processes used to use files rather than streams (hence the fields `infile` and `outfile` above; these may disappear in a later version of the ACE package).

  Each time an interactive ACE process is initiated via `ACEStart` (see ACEStart), an identifying number ioIndex is generated for the interactive process and a record `ACEData.io[`ioIndex`]` with the following fields is created. A non-interactive process has similar fields but is stored in the record `ACEData.ni`.

  `procId`
    the identifying number of the process (ioIndex for interactive processes, and 0 for a non-interactive process);

  `args`
    a record with fields: `fgens`, `rels`, `sgens` whose values are the corresponding arguments passed originally to `ACEStart`;

  `options`
    the current options record of the interactive process;

  `acegens`
    a list of strings representing the generators used by ACE (if the names of the generators passed via the first argument fgens of `ACEStart` were all lowercase alphabetic characters, then `acegens` is the `String` equivalent of fgens, i.e. `acegens[1] = String(`fgens`[1])` etc.);

  `stream`
    the IOStream opened for an interactive ACE process initiated via `ACEStart`; and

  `enumResult`
    the enumeration result (string) without the trailing newline, output from ACE;

  `stats`
    a record as output by the function `ACEStats`.

  `enforceAsis`
    a boolean that is set to `true` whenever the `asis` option (see option asis) must be set to `1`. It is usually `false`. See IsACEGeneratorsInPreferredOrder Guru Notes for the details.

- `ACEDirectoryTemporary(` dir`) F`

  calls the UNIX command `mkdir` to create dir, which must be a string, and if successful a directory object for dir is both assigned to `ACEData.tmpdir` and returned. The fields `ACEData.infile` and `ACEData.outfile` are also set to be files in `ACEData.tmpdir`, and on exit from GAP dir is removed. Most users will never need this command; by default, GAP typically chooses a ``random'' subdirectory of `/tmp` for `ACEData.tmpdir` which may occasionally have limits on what may be written there. `ACEDirectoryTemporary` permits the user to choose a directory (object) where one is not so limited.

  ## 1.9 Setting the Verbosity of ACE via Info and InfoACE

- `InfoACE V`

  The output of the ACE binary is, by default, not displayed. However the user may choose to see some, or all, of this output. This is done via the `Info` mechanism (see Chapter Info Functions in the GAP Reference Manual). For this purpose, there is the InfoClass `InfoACE`. Each line of output from ACE is directed to a call to `Info` and will be displayed for the user to see if the `InfoLevel` of `InfoACE` is high enough. By default, the `InfoLevel` of `InfoACE` is 1, and it is recommended that you leave it at this level, or higher. Only messages which we think that the user will really want to see are directed to `Info` at `InfoACE` level 1. To turn off all `InfoACE` messaging, set the `InfoACE` level to 0 (see SetInfoACELevel). For convenience, we provide the function

- `InfoACELevel() F`

  which returns the current `InfoLevel` of `InfoACE`, i.e. it is simply a shorthand for `InfoLevel(InfoACE)`.

  To set the `InfoLevel` of `InfoACE` we also provide

- `SetInfoACELevel(` level`) F`
- `SetInfoACELevel() F`

  which for a non-negative integer level, sets the `InfoLevel` of `InfoACE` to level (i.e. it is a shorthand for `SetInfoLevel(InfoACE,` level`)`), or with no arguments sets the `InfoACE` level to the default value 1. Currently, information from ACE is directed to `Info` at four `InfoACE` levels: 1, 2, 3 and 4. The command

      gap> SetInfoACELevel(2);

  enables the display of the results line of an enumeration from ACE, whereas

      gap> SetInfoACELevel(3);

  enables the display of all of the output from ACE (including ACE's banner, containing the host machine information); in particular, the progress messages, emitted by ACE when the `messages` option (see option messages) is set to a non-zero value, will be displayed via `Info`. Finally,

      gap> SetInfoACELevel(4);

  enables the display of all the input directed to ACE (behind a ```ToACE>` '' prompt, so you can distinguish it from other output). The `InfoACE` level of 4 is really for gurus who are familiar with the ACE standalone.

  ## 1.10 Acknowledgements

  Large parts of this manual, in particular the description of the options for running ACE, are directly copied from the respective descriptions in the manual Ram99 for the standalone version of ACE by Colin Ramsay. Most of the examples, in the `examples` directory and accessed via the `ACEExample` function, are direct translations of Colin Ramsay's `test###.in` files in the `src` directory.

  Many thanks to Joachim Neubüser who not only provided one of the early manual drafts and hence a template for the style of the manual and conceptual hooks for the design of the Package code, but also meticulously proof-read all drafts and made many insightful comments.

  Many thanks also to Volkmar Felsch who, in testing the ACE Package, discovered a number of bugs, made a number of important suggestions on code improvement, thoroughly checked all the examples, and provided the example found at the end of Section Steering ACE Interactively which demonstrates rather nicely how one can use the function `ACEConjugatesForSubgroupNormalClosure`.

  Finally, we wish to acknowledge the contribution of Charles Sims. The inclusion of the `incomplete` (see option incomplete) and `lenlex` (see option lenlex) options, were made in response to requests to the GAP developers to include such options for coset table functions. Also, the definition of `lenlex` standardisation of coset tables (see Section Coset Table Standardisation Schemes), is due to him.

  ## 1.11 Changes from earlier versions

  A reasonably comprehensive history of the evolution of pre-release versions of the ACE Package, is contained in the file `CHANGES` in the `gap` directory.

  The `3.xxx` versions of the ACE Package were compatible with GAP 4.2, but were built for use with GAP 4.3. However, the current version of the ACE Package requires at least GAP 4.4. Users who still have GAP 4.3 will need to use ACE 4.1. One important change in GAP from GAP 4.2 to GAP 4.3 that has relevance for ACE Package users is the change of the default standard for the numbering of cosets in a coset table (see Section Coset Table Standardisation Schemes).

- `ACEPackageVersion() F`

  gives the current version of the ACE Package (i.e. the GAP code component; the function `ACEBinaryVersion` (see ACEBinaryVersion) gives details of the C code compilation).

  History of release versions

  `3.001`
    First release.

  `3.002`
    `ACEIgnoreUnknownDefault` (see ACEIgnoreUnknownDefault) added.

    Option `continue` changed to `continu` (see option continu) -- `continue` is a keyword in GAP 4.3.

  `3.003`
    Initial value of `ACEIgnoreUnknownDefault` is now `true` (previously it was `false`).

    `ACECosetTableFromGensAndRels` and the non-interactive `ACECosetTable` (except when producing an input script via option `ACEinfile`), and the non-interactive `ACEStats` which previously used files all now invoke iostreams.

    Option `pkgbanner` used to control the printing of the banner on loading (see Loading the ACE Package).

  `4.0`
    Revised for GAP 4.4, for which the option `pkgbanner` is no longer available (most of its function is now provided by the third argument of the `LoadPackage` function; see LoadPackage).

    The package version number now has a single decimal place, to avoid being confused with the binary version number.

  `4.1`
    Added M12 presentation to `res-examples/pgrelfind.g`.

  `5.0`
    Pre-GAP 4.4 compatibility removed. Now at least GAP 4.4 is required. Option `pkgbanner` has been completely removed.

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