Exactly what it sounds like-- P-Slice for mac.
This is NOT official. I am NOT affiliated with mikolka9144, and I am CERTAINLY not associated with anyone on the Psych Engine or Funkin' teams.
This engine was originally meant to be a playground for making Psych Engine compatible with linux, but after some attempts in porting V-Slice content became a crossover between Psych Engine and V-Slice (also known as base game).
custom stickers
Mods made for P-Slice now can select which sticker pack and set to use on the next sticker transition.
setPropertyFromClass("substates.StickerSubState", "STICKER_SET", "sticker-set-1") defines which sticker set to use (it's the name of one of the directories in images/transitionSwag/)
setPropertyFromClass("substates.StickerSubState", "STICKER_PACK", "all") defines a pack of stickers to use from the current set. If such pack doesn't exist all defined stickers in a ".json" file will be used.
custom freeplay icons
You can now put V-slice freeplay icons into images/freeplay/icons/{charId}pixel.png