directories correspond to chapters in book.

      X.jrs -- snippet that appears in book
      X.jr  -- program including snippet (in most cases) to make sure
               snippet is OK (use jrv).
               can typically use jrgo to compile and run program.

  Exercises directories contain (surprise!) files related to
  exercises.  Sometimes these are as above.  In some other cases, the
  files contain "given" parts of code and data, i.e., partial programs
  from which you are to build complete programs.
