Requirements consist of (a) domain assumptions, (b) hard goals, (c) quality constraints, (d) possibly prioritized preferences. The very core of Requirements Engineering consists of the following problem: given a set of (a)-(d), generate specifications that fulfill hard goals and quality constraints, assuming that domain assumptions hold, and satisfy maximal sets of preferences. We are working towards tools that solve this problem for expressive modeling languages in terms of which one can represent domain assumptions, goals, etc. Such tools can be used as basis for exploring requirements by varying preferences and priorities, or weakening/strengthening goals.