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="" name="main">
     <p:input port="source" primary="true">
         <p:document href="BookStore.xml"/>
     <p:output port="result"/>
             <p:option name="BookStore/Book/Title" required="'false'"/>


James Garriss

Received on Friday, 12 September 2008 17:20:44 UTC