#169 Explicit sequence value

Today the sequence value of messages is implicit. It is observed from the communication time of the messages, either senders order or receivers order. The improvement is to make the sequence value explicit by making it a token with a token locator in each message.

For each Exchange add a "Sequence Token". When the sequence token is populated this is the token in the message used to identify the explicit value for the time. When this Sequence Token is empty, the sequence remains the current implicit sequence.

For example:
1. In FpML the message MessageCancelled occurs immediately upon sending and the Sequence Token is left empty.
2. In FpML the message TradeTerminated has an effectiveDate field which would be identified by the Token it's TokenLocator for the Sequence Token on the interaction.

[Gary Brown]

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At the moment I am not sure whether this should be something that is part of the core choreography model, or an annotation that is effectively an instruction to any monitoring software that indicates when the message should be considered 'received'.

So basically this could be represented using an annotation for the moment.