Copyright by Michael Fischer
11/13/2011
For use in Yale course CPSC 427a/527a, Fall term 2011.
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This is the example from section 16.4 of "Exploring C++" with some
minor updates.  It implements two linear containers of elements from
an ordered set.  List is an unordered list of elements; PQueue is an
ordered list of elements.  Both structures support deletion of
arbitrary elements.  Both are derived from an abstract class Linear.

The element type Item is an adaptor class which forms the bridge
between the generic container code and the specific element type Exam.
Item is derived from Exam.  Exam supplies a definition for typename
KeyType and method key() that are used by Item.  It also defines
method operator<<() which is inherited by Item (and can be used to
print items).  Item supplies a constructor that calls the Exam
constructor with Exam-specific arguments.

For this code to work with an arbitrary element type T, T would have
to define KeyType, key(), and print().  Item would have to derive from
T instead of from Exam.  An Item constructor appropriate to type T would
have to be defined as well.