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<Chapter Label="chap-history">
<Heading>Development history</Heading>

This chapter, which contains details of the major changes 
to the package as it develops, was first created in April 2002. 
Details of the changes from &XMod; 1 to &XMod; 2.001 are far from complete. 
Starting with version 2.009 the file <F>CHANGES</F> lists the minor changes 
as well as the more fundamental ones. 
<P/>
The inspiration for this package was the need, in the mid-1990's, 
to calculate induced crossed modules 
(see <Cite Key="BW1" />, <Cite Key="BW2" />, <Cite Key="BW3" />).
&GAP; was chosen over other computational group theory systems
because the code was freely available, and it was possible to
modify the Tietze transformation code so as to record the images
of the original generators of a presentation as words in the
simplified presentation.  
(These modifications are now a standard part of the Tietze transformation
package in &GAP;.)

<Section><Heading>Changes from version to version</Heading>

<Subsection><Heading>Version 1 for &GAP; 3</Heading>
<Index>version 1 for &GAP; 3</Index>

The first version of &XMod; became an accepted package 
for &GAP; 3.4.3 in December 1996.
</Subsection>


<Subsection><Heading>Version 2</Heading>

Conversion of &XMod; 1 from &GAP; 3.4.3 
to the new &GAP; syntax began soon after &GAP; 4 was released, 
and had a lengthy gestation.
The new &GAP; syntax encouraged a re-naming of many of the function names.
An early decision was to introduce generic names 
<C>2dObject</C> for (pre-)crossed modules and (pre-)cat1-groups, 
and <C>2dMapping</C> for the various types of morphism.
In 2.009 <C>3dObject</C> is used for crossed squares and
cat2-groups, and <C>3dMapping</C> for their morphisms.
A generic name for derivations and sections is also required, 
and <C>Up2dMapping</C> is currently used.
</Subsection>


<Subsection><Heading>Version 2.001 for &GAP; 4</Heading>
<Index>version 2.001 for &GAP; 4</Index>

This was the first version of &XMod; for &GAP; 4,
completed in April 2002 in a rush to catch the release of &GAP; 4.3.
Functions for actors and induced crossed modules were not included,
nor many of the functions for derivations and sections,
for example <C>InnerDerivation</C>.
</Subsection> 


<Subsection><Heading>Induced crossed modules</Heading>

During the period May 20th - May 27th 2002
converted <F>induce.g</F> to <F>induce.gd</F> and <F>induce.gi</F>
(later renamed <F>induce2.gd</F>, <F>induce2.gi</F>),
at least as regards induced crossed modules. 
(Induced cat1-groups may be convereted one day.) 
For details, see the file <F>CHANGES</F>.
</Subsection>


<Subsection><Heading>Versions 2.002 -- 2.006</Heading>

Version 2.002 was prepared for the 4.4 release at the end of January 2004.
<P/>
Version 2.003 of February 28th 2004 just fixed some file protections.
<P/>
Version 2.004 of April 14th 2004 added the <C>Cat1Select</C> functionality 
of version 1 to the <C>Cat1</C> function (see also version 2.007).
<P/>
Version 2.005 of April 16th 2004 moved the example files from
<F>tst/test&uscore;i.g</F> to <F>examples/example&uscore;i.g</F>,
and converted <F>testmanual.g</F> 
to a proper test file <F>tst/xmod&uscore;manual.tst</F>.
<P/>

A significant change was the conversion of the actor crossed module 
functions from the <F>3.4.4</F> version, 
including <C>AutomorphismPermGroup</C> for a crossed module, 
<C>WhiteheadXMod</C>, <C>NorrieXMod</C>, <C>LueXMod</C>, <C>ActorXMod</C>, 
<C>Centre</C> of a crossed module, <C>InnerMorphism</C> 
and <C>InnerActorXMod</C>.
</Subsection>



<Subsection><Heading>Versions 2.007 -- 2.010</Heading>

These versions contain changes made between September 2004 and October 2007. 
<List>
<Item> 
Added basic functions for crossed squares, considered as <C>3dObjects</C> 
with crossed pairings, and their morphisms. 
Groups with two normal subgroups, and the actor of a crossed module, 
provide standard examples of crossed squares. 
(Cat2-groups are not yet implemented.) 
</Item>
<Item>
Converted the documentation to the format of the &GAPDoc; package.
</Item>
<Item>
Improved <C>AutomorphismPermGroup</C> for crossed modules, 
and introduced a special method for conjugation crossed modules.
</Item>
<Item>
Substantial revisons made to <C>XModByCentralExtension</C>, 
<C>NorrieXMod</C>, <C>LueXMod</C>, <C>ActorXMod</C>, 
and <C>InclusionInducedXModByCopower</C>. 
</Item>
<Item>
Reintroduced the <C>Cat1Select</C> operation.
</Item> 
<Item> 
Version 2.010, of October 2007, was timed to coincide with the release 
of &GAP; 4.4.10, and included a change of filenames; 
correct file protection codes; and an improvement to 
<C>AutomorphismPermGroup</C> for crossed modules. 
</Item> 
</List>
</Subsection>


<Subsection><Heading>Version 2.12</Heading>
This latest version was released in November 2008. 
<List>
<Item> 
The file <C>CHANGES</C> was introduced, so that minor corrections 
need no longer be listed in this chapter. 
</Item> 
<Item> 
The file <C>makedocrel.g</C> was copied, with appropriate changes, 
from <Package>GAPDoc</Package>, and now provides the correct way 
to update the documentation. 
</Item>
<Item> 
The first functions for crossed modules of groupoids were introduced. 
</Item>
<Item>
The package webpage has moved along with the whole of the 
Bangor Maths website: <URL>http://www.maths.bangor.ac.uk/</URL>. 
</Item>
<Item>
A GNU General Public License declaration has been added. 
</Item>
</List>
</Subsection>
</Section> 



<Section><Heading>What needs doing next?</Heading>

<List>
<Item>
Speed up the calculation of Whitehead groups.
</Item>
<Item> 
Add more functions for <C>3dObjects</C> and implement <C>cat2-groups</C>.
</Item>
<Item> 
Add interaction with package <Package>Gpd</Package> implementing 
group groupoid version of a crossed module and crossed modules over groupoids. 
</Item>
<Item> 
Add interaction with <Package>IdRel</Package>, <Package>XRes</Package>, 
and <Package>natp</Package>.
</Item>
<Item> 
Need <C>InverseGeneralMapping</C> for morphisms.
</Item>
<Item> 
Need more features for <C>FpXMods</C>, <C>PcXMods</C>, etc.
</Item>
<Item> 
Implement actions of a crossed module.
</Item>
<Item> 
Implement <C>FreeXMods</C>.
</Item>
<Item> 
Implement an operation <C>Isomorphism2dObjects</C>.
</Item>
<Item>
Allow the construction of a group of morphisms of crossed modules.
</Item>
<Item> 
Complete the conversion from Version 1 of the calculation of sections
using <C>EndoClasses</C>.
</Item>
</List>

</Section>

</Chapter>