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A file-or-dir is:
 - [symbol]   (representing the name of a data file), or
 - (make-dir [symbol] [list-of-file-or-dir])

(define-struct dir (name contents))


A list-of-file-or-dir is:
  - empty
  - (cons [file-or-dir] [list-of-file-or-dir])


This defintion can represent the same
information as the 
<a href="/~comp210/01fall/Labs/lab06/">lab version</a>
and is roughly equivalent,
though 
this one more cleanly separates the list-processing
from the file-processing.

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