e_layered_create">zip_source_layered_create</code></a>()
    create a layered zip source from the user-provided function
    <var class="Ar">fn</var>, which must be of the following type:</p>
<p class="Pp"><var class="Ft">typedef zip_int64_t</var>
    <code class="Fn">(*p_source_layered_callback)</code>(<var class="Fa">zip_source_t
    *source</var>, <var class="Fa">void *userdata</var>, <var class="Fa">void
    *data</var>, <var class="Fa">zip_uint64_t length</var>,
    <var class="Fa">zip_source_cmd_t cmd