defs

states :=
  stopped,
  playing.

stopped state :=
  do nothing.

playing state :=
  play music.

play music :=
  play current song.

current song finished ->
  switch to next song.

switching to new song :=
  set current song to new song.

current, next, previous song without context :=
  song in DJ playlist.

# N is a constant which can be configured.
# If a variable is not marked as a free variable (e.g. "for all N"),
# it is a constant which has to be configured.
while num songs before current song are more than N ->
  delete first song.

while num songs after current song are less than N ->
  intelligently add song.

intelligently add song :=
  randomly select song from database,
  take liked songs more likely,
  take related song more likely.

database :=
  collection of songs.

song :=
  filesystem source,
  artist,
  name,
  album,
  genre,
  liked-state,
  last.fm-link.