Metastructures vs. Attributed Variables in the Context of Extensible Unification

We relate two mechanisms which aim at the extension of logic programming languages. The first mechanism directly extends syntactic unification through the introduction of a data type, whose (unification) semantics are specified through user-defined predicates. The second mechanism was utilized for the implementation of coroutining facilities, and was independently derived with optimal memory management for various Prolog extensions in mind. Experience from the application of both mechanisms to the realization of CLP languages, without leaving the logic programming context, enables us to reveal similarities and the potential with respect to this task. Constructive measures that narrow or close the gap between the two conceptual schemes are provided.