Differential updates (e.g. when changing parts of compositions)

[ErikSundvall]

Some openEHR implementations (in addition to posting full "canonical" instances of RM objects) allow posting compact "diffs" that use an existing version of an object (e.g. a COMPOSITION) as a base and then just supply info on what has changed.

This issue/thread was created to discuss if/how that kind of functionality could be standardized.

One thing to consider is the desired behaviour when totally omitting an attribute in a diff call. In a canonical full form that means "no value", but in a diff format it might mean "no change". Deletion in a diff-format may require supplying the attribute and setting it's value to null.

(Issue added after discussion at meeting https://openehr.atlassian.net/wiki/display/spec/2016-06-30+REST+API+Meeting+notes)