Unique property constraint

[gsvarovsky]

unique: A set of subjects in the domain (e.g. of a specific type) must have unique values for a property (besides their identity).

Conflict Scenario: Concurrent updates to two different subjects could both update the property to the same value.

Resolution: Decide the Subject to receive the conflicting value. Delete the other subject's property. If the property is mandatory, revert the value to the previous (it must exist in the same transaction).