Terminological systems are handling declarative logic-based descriptions of conceptual knowledge. Most of them restrict their expressiveness and focus on (e cient) reasoning algorithms for certain services. This imposes principal restrictions on the expressivity of such a formalism.

To get maximal bene t of terminological reasoning, while being able to overcome these expressiveness de ciencies, the terminological system Taxon has been integrated with logic programming by applying a CLP scheme to its assertional formalism. Because Taxon itself is an amalgamation of an abstract concept language by concrete domains (such as predicates over rational numbers) we have a three-layered system architecture: 1) rules on the basis of a tuned vocabulary which is formulated in 2) a concept language that is grounded by 3) concrete domains. We will discuss this layered architecture as a main prerequisite of several advantages in software/knowledge engineering, thus promoting a new discussion about principles of hybrid systems design (which is necessary when trying to gain practical relevance for declarative programming).