Issue ex-4

["Furniss, Peter" <Peter.Furniss@choreology.com>]

As I mentioned on the last call, Mark had promptly supplied the text to
complete issue ex-4, and I had
neglected to pass it on.  


Add the standard qualifier cancel-on-zero-participants

<btpq:cancel-on-zero-participants>
    <btpq:value>
        true/false
    </btpq:value>
</btpq:cancel-on-zero-participants>

With associated text:

The cancel-on-zero-participants qualifier causes a cohesion composer to
be automatically cancelled if its list of registered participants
becomes zero after either a PREPARE_INFERIORS or CANCEL_INFERIORS
message is received. This cancellation happens if the value for
cancel-on-zero-participants is set to true, and the result will be a
TRANSACTION_CANCELLED message returned to the terminator as a reply to
the PREPARE_INFERIORS or CANCEL_INFERIORS message, instead of
INFERIOR_STATUSES.

Then in the text for BEGIN we need to add something to the section on
qualifiers: the standard qualifier cancel-on-zero-participants may be
present on BEGIN for a cohesion; if the value is true, then automatic
cancellation of the cohesion will occur as per the rules for
cancel-on-zero-participants.

In the text for PREPARE_INFERIORS/CANCEL_INFERIORS for qualifers: the
cancel-on-zero-participants qualifer may be present; if it is, then it
overrides any previous value for this qualifier that may have been
provided when the cohesion was started via BEGIN.
-------------

Mark had said he didn't mind whether the default was true or false. I'm
not sure whether a default is needed. If the qualifier is absent, then
it is the same as false (existing behaviour). Do we even need a value ?

I've added the bit on the first paragraph from "terminator .." to the
end, to clarify the sequence. (Terminator:decider relationship is always
request/reply so the TRANSACTION_CANCELLED has to be a reply to
something.