UMLR — metaattribute isReadOnly

The metaattribute isReadOnly is defined both for StructuralFeature and its subclass Property. Clearly, one or the other is incorrect. This, I believe, points to an unclarity in what StructuralFeatures are, as opposed to Property. By definition, StructuralFeatures denote values that are held in slots of an object. I believe that the intuition behind Property is that a property denotes a value that might be added, modified, or deleted during the course of the lifetime of a system. This is exemplified in the two variants of property: attribute and association end. As "isReadOnly" has to do with limiting the modification of the value, it is best placed on Property, assuming this intuition is correct. This intuition is substantiated by that Port is not a property but a structural feature, and ports cannot be modified, changed, or assigned to. (Note that if this intuition is not correct, the difference between Property and StructuralFeature needs to be clarified.) A consequence of this is also that one needs to clarify the set of actions that apply to StructuralFeatures (e.g., WriteStructuralFeatureAction). If it is Properties that are modified, etc., then these actions should really apply to properties, not structural features in general. Again, this change is consistent, as none of these actions makes sense for Ports. Further, for links the actions are already limited to properties (see LinkEndData). An issue regarding the inconsistency between actions applying to structural features and actions applying to links has been submitted and should be dealt with consistently.