exercise tag is used to present a
question or task to the reader, with an optional answer hidden
from view until the reader wants to reveal it. The question or
task is contained within a problem tag and the
answer is contained within a solution tag, both of
which are inside the exercise tag.
<exercise id="exerciseexample">
<problem>
<para id="prob_1">
What is the capital of Texas?
</para>
</problem>
<solution>
<para id="sol_1">
Austin is the capital of Texas.
</para>
</solution>
</exercise>