I. Introduction
Beyond stability, the most important property to guarantee in a dynamical system is safety. The safety problem consists in guaranteeing that the system's solutions, when starting from a given set of initial conditions, never reach a given unsafe set [1]. The same property is also called conditional invariance in some earlier works [2]–[5]. Regarding the considered application, reaching the unsafe set can correspond to the impossibility of applying a predefined feedback law [6] or, simply, colliding with an obstacle [7]. Safety analysis is in fact a key step in many control applications.