The purpose of the files in this directory is to give students that
haven't finished prior labs something to build on.  At the
start of each lab, these files will bring the student up-to-date with
what they should've been starting with.  The answers (in another
directory) would do the same thing (the answer for the previous lab is
the starting point for the next), but these files also contain some
hints on what new code should be added and where that code should go.

I would probably give the students the answer code for the previous
exercise first.  And if they're having a problem with the current lab,
give them the file from this directory.  And in the lab review, cover
the answer to the lab in some detail, to ensure all topics are covered
adequately.

Note that I am still working on the answers and starting points for
the labs in Lesson 5.  The instructor's guide has some hints and tips in
Lecture 5 that I haven't turned into real code yet.

Filenames
---------
lab12.cpp
lab13.cpp
lab20.cpp
lab21a.cpp
lab21a.h
lab21b.cpp
lab21b.h
lab22.cpp
lab22.h
lab40a.cpp
lab40a.h
