LilyPond is a music typesetting system. They take their chosen problem domain very seriously. For example, in Obsessed with putting ink on paper, they explain the challenge. One of their first examples shows two different renditions of the same music, one “good”, one “bad”. To my eyes, they are identical.
I don’t need to typeset music, but I appreciate the complexity and passion of the LilyPond work. Even simple music is more complicated to set properly than text, and complex music is really complex.