Knowledge bases for real-world domains such as mechanical engineering require expressive and efficient representation and processing tools. We pursue a declarative-compilative approach to knowledge engineering. While Horn logic (as implemented in PROLOG) is well-suited for representing relational clauses, other kinds of declarative knowledge call for hybrid extensions: functional dependencies and higher-order knowledge should be modeled directly. Forward (bottom-up}.-reasoning should be

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... ed with backward (top-down) reasoning. Constraint propagation should be used wherever possible instead of search-intensive resolution. Taxonomic knowledge should be classified into an intuitive subsumption hierarchy. Our LISP-based tools provide direct translators of these declarative representations into abstract machines such as an extended Warren Abstract Machine (WAM) and specialized inference engines that are interfaced to each other. More importantly, we provide source-tosource transformers between various knowledge types, both for user convenience and machine efficiency. These formalisms with thei r translators and transformers have been developed as part of COLAB, a compilation laboratory for studying what we call, respectively, 'vertical ' and ' horizontal ' compilation of knowledge, as well as for exploring the synergetic collaboration of the knowledge representation formalisms . A case study in the realm of mechanical engineering has been an important driving force behind the development of COLAB. It will be used as the source of examples throughout the paper when discussing t he enhanced formalisms, the hybrid representation architecture, and the compilers.