Producing software variations from the same software product line requires developers to adopt developing tools that support variability. The Abstract Behavioral Specification (ABS) is a modeling language that facilitates the generation of various software products from a single code base. One part of ABS is the Product Selection Language (PSL), which is used to specify software products as sets of features. Even though some products might share some features, using PSL, all features in a product need to be stated one by one. If the product is obtained from tens to hundreds of features, defining the product will be difficult and inefficient To remedy this situation, we extend the PSL such that products can be declared incrementally, by referring to other products. Such declarations contain product expressions that use set-theoretic operations (i.e., union, intersection, complement) over products and sets of features. We evaluate our extended PSL with a case study of a Charity Organization System developed at the RSE Research Lab in the Faculty of Computer Science at Universitas Indonesia.