el filter to perform Base64 encoding.
</p>

<p class="description">
<tt>A</tt> is the encoded version of the largest prefix of
<tt>C..D</tt>
that can be encoded unambiguously. <tt>B</tt> has the remaining bytes of
<tt>C..D</tt>, <em>before</em> encoding.
If <tt>D</tt> is <tt><b>nil</b></tt>, <tt>A</tt> is padded with
the encoding of the remaining bytes of <tt>C</tt>.
</p>

<p class="note">
Note: The simplest use of this function is to encode a string into it's
Base64 transfer content encoding. Notice the ex