Does Optical Anisotropy Lead to Negative Refraction at an Interface?

This is a comment inspired by recently published results [Y. Zhang et al., Phys. Rev. Lett. 91, 157404 (2003)] that introduced the name "amphoteric refraction" for the fact that at the interface with an optically anisotropic material there can be a range of incidence angles for which the component of the Poynting vector parallel to the interface changes sign upon refraction. The latter effect is a well-known consequence of optical anisotropy, but it was described as a new negative refraction phenomenon that can be put in the same class as the negative refraction observed at an interface with a left-handed material with negative refractive index.