FL-systems are conditional rewriting systems. They are used for programming (describing) and evaluating (generating) huge 3D virtual environments, such as cities and forests. This paper presents a formal semantics and a dynamic dependency analysis for FL-systems. This analysis allows the characterization of a set of terms which are joinable with the currently rewritten term. Consequently, it is possible to speed up the rewriting steps of the environments generation by using a cache mechanism which is smarter than standard ones. This work can be seen as a dynamic completion of a set of rewriting rules. This completion increases the number of terms which are rewritten in normal form by the application of a single rewriting rule.