If the ChurchOfEmacs is too religious for you, try the secular variant
-- "Comparative Emacsology".

Comparative Emacsology studies the incompatible differences in Emacs
variants and how individuals and groups of developers strive to make
their EmacsLisp code portable.  One prime example for such differences
is EmacsOverlays vs. XEmacsExtents.  Most authors dealing with any
non-trivial highlighting will have to write compatibility layers for
such issues.  Another example is keybindings: there are several
ways to specify a key, some of them deprecated, some of them "official"
since they are part of the EmacsManual, and then there are various
compatibility notations.

Truly a field where big and important problems await study and remedy.

Some of the things we already looked at:

* AddKeywords
* CompiledFile
* CustomizingBoth
* ElispCompatibility
* EmacsAndXEmacs
* EnablingFontLock
* FreeVariable
* LineInformation
* MouseEvent
* SplitString
* TemporaryFaces
* TheaterStyle
* WindowHeight
* XEmacsExtents

Some notes in table format. If you a lot to add, remove the row from the list and put the issue on a separate page.

|| *Emacs* || *XEmacs* || *Note* ||
|| bool-vector || bit-vector || -- ||
|| integers || characters || Emacs uses plain integers: (eq ?A 65) => t ||
|| derived-mode-p || -- || Use the one in Emacs' subr.el and rename it to fit your package's namespace. ||
|| char-valid-p || characterp || -- ||
|| line-number-at-pos || line-number || -- ||
|| balance-windows || balance-windows || Doesn't balance vertical windows in XEmacs. ||

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