Exploring problems through multiple alternatives is a key aspect of design. In this paper, we present a prototype system as an extension to existing parametric CAD tools that enables parallel generation and editing of design alternatives. The system is built on two fundamental ideas. First, use of subjunctive dependency graphs enables simultaneous work on multiple design variations. These graphs capture and reveal complex data flow across alternative parametric CAD models. Second, prototype-based modeling provides a weak notion of inheritance enabling incremental description of differences between alternatives. The system is intended to be general enough to be used in different CAD platforms and other systems using graph-based modeling. The three basic system functions are definition of alternatives (variations) using prototype-based modeling, structural and parametric divergences of the prototypes, and interactive comparison. The goal of this research is consistent with the general qualities expected from any creativity support tools: enabling exploration and simultaneous development of variations.