Duties of a binding

Falling out of the previous message, here's what a binding of the
one-way MEP has to do:

    * Given a destination, determine the set of receivers.
    * For each receiver, specify the SOAP message path.
    * For each receiver, ensure that the received message is the same as
      the sent message (unless there's a failure, in which case ...)

That looks to be it.  Do that and you can make small tools that fit
together nicely.