I think you have indeed detected an error. The element <castList> is a
member of the classes comp.drama and inter, but not a member of
dramafront. This means that it cannot appear directly within a <front>
element, but must be contained by some other element, such as a <div> or
a <performance> element.
I'm away from my home machine at present, so this is based on my reading
of the Guidelines rather than experimental evidence.
Of course, it remains to be determined whether it is the DTD that is at
fault (i.e. whether castList should be permitted to appear directly
inside <front> (as it is within <body>) or the documentation (i.e. the
examples in P3 that have mislead you)
Thanks for reporting this one!