Proposal for issue 3104 - exchange types

Proposal for issue: http://www.w3.org/Bugs/Public/show_bug.cgi?id=3104


Section 6.2.3 - Interaction syntax

Need to add bullet point:

"The type of the exchange, as described in the |informationType| or 
|channelType| attributes, must be defined if the channel type associated 
with the interaction has identity tokens specified."