stability
Calculates the next stability for items that have lapsed (i.e., rated as AGAIN) based on the current difficulty, stability, and retrievability.
Return
The next stability value, which indicates how long until the item should be reviewed again.
Parameters
difficulty
The current difficulty of the item. (1.0 to 10.0)
stability
The current stability of the item. (in days)
retrievability
The current retrievability of the item. (0.0 to 1.0)