Metaterms with Several Attributes

Metastructures and metaterms have been proposed to extend Prolog systems 6, 5]. They enable special purpose uniications to be speciied. They are useful to implement for example coroutining and new domains of computation. In both cases, information is attached to variables, which then are encoded with metaterms or metastructures. This information must be taken into account during uniication. Problems appear when one wants to use metaterms for diierent issues: how to unify metaterms of diierent kinds with a single uniication handler? These problems especially appear in a modular programming with independent pieces of code, where it is impossible to write a single handler for all uses of metaterms hidden in modules. In this paper, we propose a solution where there is only one kind of metaterms which has several facets, each facet serving a special issue. Then each facet is independently uniied. This solution has been implemented in the ECRC Prolog system ECL i PS e .