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# 10.1 Concept

There are two main changes in the concept. The first is the migration to GAP4 with all the bells and whistles like object oriented design with operations, methods and method selection via filters. XGAP4 is rewritten nearly totally with these technologies. This should make the reusage of code in the future easier. One can now use big parts of the code of XGAP for own structures by just replacing some methods via overloading.

The second change is that there is no longer any mathematical ``knowledge'' or algorithm in XGAP. It is now only a front end and a graphical user interface. All code for finitely presented groups resides now in the GAP library. This is a much cleaner concept and should make the management of the source code easier. At the same time XGAP has become a much more generic program. Operations for subgroups are for example no longer hard wired into XGAP case by case but there is generic code which can be adapted just by hacking a few tables.

These generalizations made some sacrifices necessary, because XGAP does no longer know anything about the mathematics it is displaying. It may for example happen that XGAP does no longer adapt its behaviour to the amount of data that is known about some finitely presented groups. The reason for this is, that the generic poset routines cannot know that the vertices stand for groups at all. So sometimes one has to trigger the comparison of subgroups of finitely presented groups manually (see section Compare Subgroups for a description how to do this).

In the old GAP3 version of XGAP there were three different programs for the full subgroup lattice of a (finite) group (`GraphicLattice`), the interactive partial subgroup lattice of a finite group (`InteractiveLattice`) and the interactive partial subgroup lattice of a finitely presented group (`InteractiveFpLattice`) respectively. Now there is only one generic program to display subgroup lattices interactively (`GraphicSubgroupLattice`).

XGAP can now handle subgroups of infinite index. They are either placed in a ``finite size'' level or in an ``infinity'' level. See levelsintro for details.

A new logging facility allows to automatically produce a protocol of the actions the user performs via mouse clicks. This is convenient because the normal GAP command script contains no useful information about the selected entries in the menus. See loggingfacility for details.

There is a new layer to display generic posets that do not have to be subgroup lattices. It can be used to display posets interactively very easily. This is for example used in the new link to the C-MeatAxe written by Michael Ringe. The code for this link is also included in XGAP4.

Code for the display of graphic graphs is planned but not yet completed.

The user of XGAP should not realize much of those changes (except of course the name of the function to display a subgroup lattice). The programmer on the other hand has to get used to the new techniques. It was not in all places possible to achieve total compatibility for existing code. Some changes also were introduced deliberately to make the programmers adapt their programs to the new situation!

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