UMLR — Abstraction::mapping should be of type ValueSpecification or OpaqueExpression

In many cases, modeler would like to specify the mapping of an Abstraction on an informal level by just providing a LiteralString or OpaqueExpression describing the mapping in a natural language.

The necessity to use an Expression is for this kind of usage of this feature cumbersome and clunky.

The resolution could be to use a more common metaclass of Expression, i.e. ValueSpecification, to provide the highest level of flexibility to the modeler, how mappings can be specified.