concepts:
- learning targets
- can relate to other concepts in the following ways:
  - concept A "depends" on concept B; explaining concept A requires some information from concept B
  - concept A "optdepends" on concept B
  - concept A "satisfies" concept B; mastery of concept A implies mastery of concept B

topics:
- groups of concepts
- can nest infinitely

"reviews" are randomly constructed sets of activities

each user has a mastery level for each concept

references:
- super memo algorithm used by anki: https://en.wikipedia.org/wiki/SuperMemo#Description_of_SM-2_algorithm