TeachScheme! 2002

 whatever they want to work on.

  give (cond [Q1 A1]
             [else A2])
   to those who finish interp-with-defs; encourage to interpret
   factorial with their evaluator

  For those who finished the COND, we had them ensure that their
  evaluators could handle mutual recursion.  EVEN?/ODD? is a good
  example, since it requires no new machinery.