o be tested without having to check for an undefined
value.</p>

<blockquote><table class='norm'><tr><th>Name</th><th>Description</th></tr>

<tr><td>ADD_DIRS</td><td>Hash of parent directories for each directory added during
    writing (Note: entry may exist but be undefined for top-level
    directory!)  To avoid adding a directory twice, entries may be
    deleted from this lookup, but only AFTER WriteDirectory() is called.</td></tr>

<tr><td>BASE</td><td>Absolute position of current image in containing 