2 questions about p:store

A pipeline that contains this:

    <p:for-each name="step1">
        <p:iteration-source select="//Book"/>
        <p:store href="books.xml"/>
    </p:for-each>


might overwrite the file "books.xml" many times.

1) Does the the XProc spec give guidance to XProc processors on how to
handle overwrites?  Should it?

2) How would you prevent an overwrite?  Is there a way to give a unique
filename for each book when p:store writes it?

TIA,

James Garriss
http://garriss.blogspot.com