Many recent proposals for the integration of functional and logic programming use conditional term rewriting systems (CTRS) as programs and narrowing as goal solving mechanism. This paper speci es a computation strategy for lazy conditional narrowing, based on the idea of transforming patterns into decision trees to control the computation. The speci cation is presented as a translation of CTRS into Prolog, which makes it executable and portable. Moreover, in comparison to related approaches, our method works for a wider class of CTRS.