Where
<replaceable>123456
</replaceable> is a number that is exactly the same as the number in the existing
<literal>c:
</literal> entry for size. Basically you are duplicating the existing
<literal>c:
</literal> line as an
<literal>a:
</literal> line, making sure that fstype is
<literal>4.2BSD
</literal>. Save the file and exit.