NEW ISSUE: Title: Use of SchemaTron to formalize usage constraints

TARGET: SCA C+I WS-BPEL spec, General

DESCRIPTION: There are several places in the document where we make 
statements limiting the usage of our defined XML items.  An example from 
Section 3.3, where sca-bpel:service and sca-bpel:reference are defined:
"A process MUST NOT include both attributes on a single partnerLink."  
This kind of restriction can not be encoded in XML Schema intuitively, 
so we don't.  Furthermore, there is an implicit restriction that these 
attributes are only to be used on a partnerLink, but again, we don't 
state that formally.

PROPOSAL: Define SchemaTron rules in our specification that formalize 
the constraints that are now limited to either informal verbal 
constraints, or implied constraints.