  The class of skewon-axion media can be defined in a simple and natural manner
applying four-dimensional differential-form representation of electromagnetic
fields and media. It has been recently shown that an interface of a uniaxial
skewon-axion medium acts as a DB boundary requiring vanishing normal components
of the D and B vectors. In the present paper a more general skewon-axion medium
is considered. It is shown that a planar interface of such a medium acts as a
boundary generalizing both soft-and-hard (SH) and DB boundary conditions to
SHDB conditions. Reflection of a plane wave from a planar SHDB boundary is
studied. It is shown that for the two eigenpolarizations the boundary can be
replaced by equivalent PEC or PMC boundaries. The theory is tested with a
numerical example.
