In this section, we discuss in more details problems listed in the conclusion of 4_sec:generic_interface and related to some important abstract operators. There are two main categories of problems: the differences of signatures between existing implementations for an operator, such as naming conventions, returned codes; and the availability of some implementations. We begin with an easy operator that returns a boolean answer, the test of emptiness of a HQSet, i.e. the satisfiability of the constraint system used to define it.