GAP 4.8.9 installation with standard packages -- copy to your CoCalc project to get it

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  1 Introduction
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  The XMod package provides functions for computation with
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      finite  crossed  modules  of  groups and cat1-groups, and morphisms of
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        these structures;
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      finite   pre-crossed   modules,  pre-cat1-groups,  and  their  Peiffer
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        quotients;
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      derivations of crossed modules and sections of cat1-groups;
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      isoclinism of groups and crossed modules;
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      the actor crossed square of a crossed module;
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      crossed   squares,  cat2-groups,  and  their  morphisms  (experimental
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        version);
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      crossed modules of groupoids (experimental version).
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  It is loaded with the command
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    Example  
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    gap> LoadPackage( "xmod" ); 
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  The  term  crossed  module  was introduced by J. H. C. Whitehead in [Whi48],
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  [Whi49]. Loday, in [Lod82], reformulated the notion of a crossed module as a
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  cat1-group.  Norrie  [Nor90],  [Nor87]  and  Gilbert  [Gil90]  have  studied
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  derivations,  automorphisms  of  crossed  modules and the actor of a crossed
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  module,  while  Ellis [Ell84] has investigated higher dimensional analogues.
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  Properties of induced crossed modules have been determined by Brown, Higgins
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  and Wensley in [BH78], [BW95] and [BW96]. For further references see [AW00],
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  where  we  discuss  some  of the data structures and algorithms used in this
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  package, and also tabulate isomorphism classes of cat1-groups up to size 30.
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  XMod  was  originally implemented in 1997 using the GAP 3 language. In April
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  2002  the  first and third parts were converted to GAP 4, the pre-structures
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  were  added,  and  version 2.001 was released. The final two parts, covering
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  derivations,  sections and actors, were included in the January 2004 release
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  2.002  for  GAP 4.4. Many of the function names have been changed during the
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  conversion, for example ConjugationXMod has become XModByNormalSubgroup. For
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  a list of name changes see the file names.pdf in the doc directory.
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  In  October  2015  Alper  Odabaş  and  Enver  Uslu were added to the list of
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  package  authors. Their functions for computing isoclinism classes of groups
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  and  crossed modules are contained in Chapter 4, and are described in detail
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  in their paper [IOU16].
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  The current version is 2.64 for GAP 4.8, released on 30th November 2017.
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  The package may be obtained as a compressed tar file xmod-2.64.tar.gz by ftp
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  from one of the following sites:
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      any                  GAP                 archive,                 e.g.
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        http://www.gap-system.org/Packages/packages.html;
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      the                            Bangor                            site:
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        http://www.maths.bangor.ac.uk/chda/gap4/xmod/xmod.html;
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      the package GitHub repository: https://github.com/gap-packages/xmod.
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  Crossed  modules  and  cat1-groups are special types of 2-dimensional groups
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  [Bro82],   [BHS11],   and   are   implemented   as  2DimensionalDomains  and
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  2DimensionalGroups having a Source and a Range.
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  The  package  divides into eight parts. The first part is concerned with the
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  standard constructions for pre-crossed modules and crossed modules; together
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  with  direct products; normal sub-crossed modules; and quotients. Operations
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  for constructing pre-cat1-groups and cat1-groups, and for converting between
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  cat1-groups and crossed modules, are also included.
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  The  second  part  is  concerned with morphisms of (pre-)crossed modules and
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  (pre-)cat1-groups,  together with standard operations for morphisms, such as
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  composition, image and kernel.
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  The third part is the most recent part of the package, introduced in October
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  2015.  Additional operations and properties for crossed modules are included
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  in  Section  4.1. Then, in 4.2 and 4.3 there are functions for isoclinism of
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  groups and crossed modules.
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  The fourth part is concerned with the equivalent notions of derivation for a
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  crossed module and section for a cat1-group, and the monoids which they form
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  under the Whitehead multiplication.
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  The  fifth  part deals with actor crossed modules and actor cat1-groups. For
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  the  actor  crossed  module  Act(mathcalX)  of  a crossed module mathcalX we
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  require  representations  for  the Whitehead group of regular derivations of
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  mathcalX  and  for  the group of automorphisms of mathcalX. The construction
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  also  provides an inner morphism from mathcalX to Act(mathcalX) whose kernel
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  is the centre of mathcalX.
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  The  sixth  part,  which  remains  under  development, contains functions to
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  compute induced crossed modules.
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  Since version 2.007 there are experimental functions for crossed squares and
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  their morphisms, structures which arise as 3-dimensional groups. Examples of
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  these  are  inclusions of normal sub-crossed modules, and the inner morphism
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  from a crossed module to its actor.
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  The  eighth  part  has  some  experimental  functions for crossed modules of
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  groupoids, interacting with the package groupoids. Much more work on this is
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  needed.
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  Future  plans  include the implementation of group-graphs which will provide
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  examples   of   pre-crossed   modules  (their  implementation  will  require
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  interaction  with  graph-theoretic functions in GAP 4). There are also plans
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  to implement cat2-groups, and conversion betwen these and crossed squares.
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  The  equivalent categories XMod (crossed modules) and Cat1 (cat1-groups) are
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  also  equivalent  to GpGpd, the subcategory of group objects in the category
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  Gpd  of  groupoids.  Finite  groupoids have been implemented in Emma Moore's
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  package groupoids [Moo01] for groupoids and crossed resolutions.
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  In  order  that  the  user  has  some  control  of the verbosity of the XMod
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  package's functions, an InfoClass InfoXMod is provided (see Chapter ref:Info
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  Functions  in  the  GAP  Reference  Manual  for  a  description  of the Info
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  mechanism).  By  default, the InfoLevel of InfoXMod is 0; progressively more
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  information is supplied by raising the InfoLevel to 1, 2 and 3.
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    Example  
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    gap> SetInfoLevel( InfoXMod, 1); #sets the InfoXMod level to 1
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  Once  the  package  is loaded, the manual doc/manual.pdf can be found in the
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  documentation  folder. The html versions, with or without MathJax, should be
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  rebuilt as follows:
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    Example  
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    gap> ReadPackage( "xmod, "makedocrel.g" ); 
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  It  is  possible  to  check that the package has been installed correctly by
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  running the test files:
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    Example  
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    gap> ReadPackage( "xmod", "tst/testall.g" );
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    #I  Testing .../pkg/xmod/tst/gp2obj.tst 
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    ... 
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  Additional  information can be found on the Computational Higher-dimensional
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  Discrete                 Algebra                 website                 at:
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  http://pages.bangor.ac.uk/~mas023/chda/intro.html.
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  This  version  2.61  had  to  be  released  in a hurry when required package
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  groupoids  was  renamed as groupoids. As a result some functions for crossed
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  squares  and  cat2-groups,  still  under  development, had to be temporarily
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  removed,  and  two test files had to be removed from the list of files to be
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  tested.
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