Naming convention for exercises, problem sets, etc

The most general groups of "things for students to do that are not videos", the ones clickable from a link on the course home page, are "problem sets".

Each Problem set

• contains one-or-many "exercises".
• is represented by a "stack" of cards.
• is described in one "summative" khan-exercises format file (it's basically a special case of an exercise).

Each Exercise

• contains one-or-many "problems", variations out of which only one will be randomly displayed
• is represented by one card in the stack
• is described in a single khan-exercises format file

Each Problem

• is a variation for the exercise
• is described in a child of the
  element in the exercise file.
• may or may not get displayed

Now, when problem sets are rendered, one problem is actually chosen from each exercise belonging to the problem set. So while rendered in javascript problem sets have problem children. But in the html sources problem sets only have exercise children.