Metasurfaces (MSs) are the 2-D equivalent of metaterials, the latter being a class of artificial engineered materials exhibiting peculiar electromagnetic properties [1], [2]. Generally constructed as arrays of deeply sub-wavelength resonant particles on a substrate, the geometry of the particles can be carefully designed to produce transformations of incident waves, including control of phase, amplitude, polarization, and direction of propagation. Recently, a major research direction has been the "intelligent" Metasurface (MS), where the wave transformation can be electrically controlled, which could be a viable means of achieving, in 5G and future wireless communications, the goal of manipulating and optimizing the propagation environment [3] or to even create sophisticated illusions and holograms on the fly [4]. Other topics that have recently been studied are surfaces with time-varying properties [5] and surfaces composed of particles with multipolar moments [6].