Hilchot Shabbat 11:13-14 (Writing in an unusual way, and editing an existing letter)
11:11 am
The Rambam taught that someone who writes two letters on Shabbat is liable, but is exempt for writing only one letter. Today he explains that editing a single letter so that it becomes two letters makes a person liable, as well. But what does the Rambam say about editing a single letter so that it becomes a different letter rather than two letters? The answer is not obvious.