  The paper presents the general concept of spacetime processing metasurfaces,
synthesized by generalized sheet transition conditions (GSTCs). It is shown
that such metasurfaces can perform multiple simultaneous spatio-temporal
processing transformations on incident electromagnetic waves. A time-reversal
space-generalized-refraction metasurface and a multi-time-space-differentiating
metasurfaces are presented as applications of the general spacetime processing
metasurface concept.
