Domain-specific languages (DSLs) have been proposed as a solution to ease the development of programs within a program family. Sometimes, however, experience with the use of a DSL reveals the presence of subfamilies within the family targeted by the language. We are then faced with the question of how to capture these subfamilies in DSL abstractions. A solution should retain features of the original DSL to leverage existing expertise and support tools.The Bossa DSL is a language targeted towards the development of kernel process scheduling policies. We have encountered the issue of program subfamilies in using this language to implement an encyclopedic, multi-OS library of scheduling policies. In this paper, we propose that introducing certain kinds of modularity into the language can furnish abstractions appropriate for implementing scheduling policy subfamilies. We present the design of our modular language, Bossa Nova, and assess the language quantitatively and qualitatively.