ZK | ◊ repeat wrong items four trials later

part of the StudySmarter SR algo
alternative to repeating wrong items after n seconds
⇄ time-based vs. trial-based learning item scheduling
doing this now in ⌬ karten, kind of (new cards are first freely memorized, than integrated into normal SR flow, but blocked for four ◊ trial or so, because otherwise would be weird and boring)