Sly is a program written in plain text in the python language. It's purpose is to make data entry for the LilyPond music typesetting program easier. A LilyPond music file which contains all its notes is often huge and unwieldy for editing. If you have many parts, it is very difficult to keep them all aligned when they are spread out in the text. Sly is a solution. Writesly writes a new sly file from the parts listed in the header. Check out this fine manual for sly..