Typically, one starts with the "neutral class" formula_48, containing a single object of size 0 (the "neutral object", often denoted by formula_49), and one or more "atomic classes" formula_50, each containing a single object of size 1. Next, set-theoretic relations involving various simple operations, such as disjoint unions, products, sets, sequences, and multisets define more complex classes in terms of the already defined classes. These relations may be recursive. The elegance of symbolic combinatorics lies in that the set theoretic, or "symbolic", relations translate directly into "algebraic" relations involving the generating functions.