Complex sentence: Zawinski and the other programmers began making fundamental changes to GNU Emacs to add new functionality; tensions over how to merge these patches into the main tree eventually led to the famous fork of the project into GNU Emacs and XEmacs.
Split into simpler sentences: Zawinski and the other programmers made fundamental changes to GNU Emacs to add new functionality. Tensions over how to merge these patches into the main tree eventually led to the fork of the project into GNU Emacs and XEmacs.