UML22 — Actions on non-unique properties with location specified

Actions on non-unique properties with location specified. Clarify what happens in the actions applying to non-unique features / association ends when they specify location and an existing value (eg, RemoveStructuralFeature and Destroy actions) if the value to be acted on is not at the position specified.

Currently, the WriteStructuralFeatureAction and WriteVariableAction superclasses specify a required value input pin, so all kinds of write structural feature actions must have such a pin in all cases. However, when a removeAt pin is required for a RemoveStructuralFeatureValueAction or RemoveVariableValueAction (that is, when the feature or variable is ordered and non-unique and isRemoveDuplicates is false), the expectation is that whatever value is at the given position is removed. Having to provide any value at all is counterintuitive. If the value is ignored, then it is pointless. If the value has to be the same as the value at the given position, then it is extremely inconvenient and redundant to have to read the value at that position just to remove it!
Therefore, the remove value actions should not have a value input pin in the case they are required to have a removeAt pin. This means that the value input pin should be optional in the write action superclasses, but should then be constrained to be required for the add value actions.