 in or expected to be
    returned, or else <code class="Dv">NULL</code> and 0. The last argument,
    <var class="Ar">cmd</var>, specifies which action the function should
    perform.</p>
<p class="Pp">See
    <a class="Xr" href="zip_source_function.html">zip_source_function(3)</a> for
    a description of the commands.</p>
<p class="Pp">A layered source transforms the data or metadata of the source
    below in some way. Layered sources can't support writing and are not
    sufficient to cleanly add supp