[CAP] update segments

[Mick Jagger]

> Alerts should always be tied to a specific location. An update to an alert that subtracts the original location and adds two more is not an update but a new alert.
> 

Hi,
	This brings up the issue of what should and shouldn't be changed in an update message.  If a new area is added to an original alert, does it become a new alert and just reference the original?  How about changes in urgency/severity/certainty that downgrade an alert?  Maybe the addition of a new resource value?  Or do all fields of an initial alert get "locked" and only the descriptive fields such as description/instruction change?

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