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# 1 Preface

### Sections

1. Authors

The XMod package provides functions for the computation with

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

It is loaded with the command

    gap> RequirePackage( "xmod" );

XMod was originally implemented in 1997 using the GAP 3 language. The first two parts have been converted to GAP 4. The parts covering derivations; sections and actors will follow in due course. Many of the function names have been changed during the conversion, for example `ConjugationXMod` has become `XModByNormalSubgroup`.

- `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 levels of `1`, `2` and `3`, e.g.

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

  ## 1.1 Authors

  XMod has been developed by

  Murat Alp
    Dumlupinar Universitesi, Fen-Edebiyat Fakultesi, Matematik Bolumu,
    Merkez Kampus, Kutahya, Turkey.
    e-mail: malp@mail.dumlupinar.edu.tr

  Chris Wensley
    School of Informatics, University of Wales Bangor,
    Dean Street, Bangor, Gwynedd, LL57 1UT, U.K.
    e-mail: c.d.wensley@bangor.ac.uk

  Please send bug reports, suggestions and other comments to the second of these e-mail addresses.

  Additional information can be found on the Computational Discrete Algebra web site at

  http://www.informatics.bangor.ac.uk/public/math/research/hdda/

  Copyright copyright 2002 by Murat Alp and Chris Wensley.

  XMod is subject to the GAP copyright regulations as detailed in the copyright notice in the GAP manual.

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