All tags that implement a foil structure have an optional arg of max
that controls the maximum number of total foils to show.
- foilgroup: required. Must surround all
- foil: required. The foil is defined by what
is delimited by the foil tag.
- conceptgroup: optional. Surrounds a
collection of foil. When a problem is displayed, only one
of the contained foil is selected for display. It has one
required attribute concept.