+* LEVEL PROGRESSION
+
+Neverball levels are grouped in level sets. All sets are immediately
+accessible to the player, and track of progress is kept separately for
+each set. The player starts out at the first level and must complete
+each level in turn to unlock subsequent levels until all levels in the
+set are completed. A level is "unlocked" by completing the previous
+level in the progression or if it is the first level of the set. A
+level is "completed" by collecing the required amount of coins to
+activate the goal in the level and reaching said goal.
+
+There are two game modes or ways of progressing through levels in
+Neverball. Each mode is essentially a subset of the single way of
+playing present in Neverball 1.4.0.
+
+In 1.4.0, the game keeps track of "lives" or balls, preventing a level
+from being retried once the balls run out, however, unlocked/completed
+levels remain accessible in the menu at all times. As a special case,
+1.4.0 also records a set high-score (see below for information on
+high-scores) when all levels of a set are played through in one
+attempt, with the condition that no successfully-completed level be
+restarted during the run.
+
+To address some of the problems with this mode of playing, in
+Neverball 1.5.0 it was split into two separate modes: the "normal"
+mode and the challenge mode.
+
+In the normal mode, no track of balls is kept. Each
+unlocked/completed level is immediately accessible and can be retried
+and restarted mid-game as often as desired.
+
+In the challenge mode, the player is given a limited number of balls
+and starts out at the first level of the set with all other levels
+locked. The game ends once the balls run out or all levels in the set
+are completed. A set score is recorded for a successful run. Levels
+cannot be restarted, but can be retried.
+
+In addition to regular 1.4.0-style levels, Neverball 1.5.0 introduces
+bonus levels: extra levels awarded for playing challenge mode. Bonus
+levels can only be unlocked by completing all the levels before them
+in challenge mode, however, bonus levels themselves are skipped in
+challenge mode; after they have been unlocked, they can be played in
+the normal mode as any regular level.
+
+