<li>F-cards in a directory artifact may not have directory separator
characters in their filename.  The F-cards of a directory specify only
the files and subdirectories in that directory.
<li>F-cards in a directory artifact may use the "d" permission character
to indicate that the entry refers to a subdirectory.  In that case, the
SHA1 hash on the second argument refers to another directory artifact,
not to a content artifact.
<li>F-cards in a directory artifact must have at least two arguments.
</ul>

The Z-card in a directory artifact is required and has the same format as
it does in all other special artifacts.  Directory artifacts may not be
PGP-clearsigned.

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