compound steps and primary inputs

Do all compound steps have primary inputs?  I ask because I can't tell by
reading the WD if p:for-each has a primary input or not.  I'm probably
missing a key word somewhere.

When I use it in Calabash 0.6.1, it behaves as if it does:

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc">
    <p:input port="source">
        <p:document href="BookStore.xml"/>
    </p:input>
    <p:output port="result"/>
    <p:filter select="/BookStore/Book[Date>'1970']"/>
    <p:for-each>
        <p:output port="result"/>
        <p:identity/>
    </p:for-each>
</p:declare-step>

The output of filter, a sequence of documents (<Book>...</Book>), is being
inputted (is that a word?) into p:for-each.

Appreciate any insights!

James Garriss
http://garriss.blogspot.com