t>A</tt> is a copy of <tt>B</tt>, broken into lines of at most
<tt>length</tt> bytes (defaults to 76).
'<tt>n</tt>' should tell how many bytes are left for the first
line of <tt>B</tt> and '<tt>m</tt>' returns the number of bytes
left in the last line of <tt>A</tt>.
</p>

<p class="note">
Note: This function only breaks lines that are bigger than
<tt>length</tt> bytes. The resulting line length does not include the CRLF
marker.
</p>


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