“…In the case when the system stems from physical modeling, the storage function is the energy of the system and the product has then the dimension of power. This property can be described by a Dirac structure [4], [22]; to be precise, it satisfies a Stokes-Dirac structure on jet bundles [1], [6], [8], [15]. This means that (4) has an integrable quantity given by Stokes's theorem; actually, (4) is characterized by the power balance equation…”