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# 1 Preface

The XMod package provides functions for the computation with finite

- crossed modules and cat1-groups, and morphisms of these structures;
- pre-crossed modules, pre-cat1-groups, and their Peiffer quotients;
- derivations of crossed modules and sections of cat1-groups; and
- the actor crossed square of a crossed module.

It is loaded with the command

    gap> LoadPackage( "xmod" );

XMod was originally implemented in 1997 using the GAP 3 language. In April 2002 the first and third parts were converted to GAP 4, the pre-structures were added, and version 2.001 was released. The final two parts, covering derivations, sections and actors, were included in the January 2004 release 2.002 for GAP 4.4.

The current version is 2.006, released on September 6th 2004.

Many of the function names have been changed during the conversion, for example `ConjugationXMod` has become `XModByNormalSubgroup`. For a list of name changes see the file `names.pdf` in the `doc` directory.

- `InfoXMod V`

  In order that the user has some control of the verbosity of the XMod package's functions, an InfoClass `InfoXMod` is provided (see Chapter Info Functions in the GAP Reference Manual for a description of the `Info` mechanism). By default, the `InfoLevel` of `InfoXMod` is `0`; progressively more information is supplied by raising the `InfoLevel` to `1`, `2` and `3`, e.g.

      gap> SetInfoLevel( InfoXMod, 1); #sets the InfoXMod level to 1

  The following test file (from the ``xmod'' directory) runs all the manual commands.

      gap> ReadTest("tst/xmod_manual.tst");
      + Testing constructions of crossed modules and cat1-groups
      + GAP4stones: 0
      true

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