step must contain only a signature

I'm trying to write some simple pipelines with XProc.  I seem to get  
this error frequently for various steps (swap out <p:delete> for  
whatever step I'm using):

Pipeline failed: org.xproc.XProcException: A p:delete step must  
contain only a signature.

What is this error trying to tell me?  I suspect it's telling me I  
have the wrong number of inputs and outputs, but I don't quite get  
what I'm missing (or adding).

For grins, here's my pipeline:

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="main">
     <p:input port="source" primary="true">
         <p:document href="BookStore.xml"/>
     </p:input>
     <p:output port="result"/>
     <p:pipeline>
         <p:delete>
             <p:option name="BookStore/Book/Title" required="'false'"/>
         </p:delete>
     </p:pipeline>
</p:declare-step>

TIA,

James Garriss
http://garriss.blogspot.com