J S Bach, Partita 3, BWV 1006 https://imslp.org/wiki/File:PMLP04292-Partita_BWV_1006.PDF
could be used as exercise for nested (slow)cat.
example: measure 20 and 21:
<bf*12 a*12>/12 <f <d*3 c*3 c*3 bf4*3>/3 >
Ugh, it seems that IMSLP does not allow "deep" links (I guess they want/need to show the copyright disclaimer first. Well, then go through https://imslp.org/wiki/6_Violin_Sonatas_and_Partitas,_BWV_1001-1006_(Bach,_Johann_Sebastian) ),
or https://www.mutopiaproject.org/ftp/BachJS/BWV1006/bwv-1006_1/ , they also have a MIDI file (generated from lilypond, I guess) that one could use as input for an automatic pattern compressor ...
And, the long game is this: once you have a compressed representation C of (some piece of music) M, you change one small piece in C, and get back some M' that is somehow similar to M, but also somehow not. (Try with the above, change any note in the pattern.) That's what LLMs do, sort of. (M = training data, C = weights, difference(C',C) = prompt, hand-wavingly)