a</code> writes a nested list with each sublist on one line.
</p>
<p><code>write_data</code> writes a flat list all on one line.
</p>
<p>The destination <var>D</var> may be a file name or a stream.
When the destination is a file name,
the global variable <code>file_output_append</code> governs
whether the output file is appended or truncated.
When the destination is a stream,
no special action is taken by <code>write_data</code> after all the data are written;
in particular, the stream remains open.
</p>
<p