Ubiquitous computing increases the pressure on the software industry to produce ever more and error-free code. Two recipes from automated programming are available to meet this challenge: On the one hand, generative programming raises the level of abstraction in software development by describing problems in high-level domain-specific languages and making them executable. On the other hand, in situations where one needs to produce a family of similar programs, product line engineering supports code reuse by composing programs from a set of common assets (or features). AHEAD (Algebraic Hierarchical Equations for Application Design) is a framework for generative programming and product line engineering that achieves additional productivity gains by scaling feature composition up. Our contribution is GRAFT, a calculus that gives a formal foundation to AHEAD and provides several mechanisms for making sure that feature combinations are legal and that features in themselves are consistent.