Convert an iterable slicespec (supplying r1,r2,c1,c2 of a
        Slice) into a BoundingRegion specification.

        Exact inverse of _boundsspec2slicespec().

def _slicespec2boundsspec(slicespec,scs):
        """
        Convert an iterable slicespec (supplying r1,r2,c1,c2 of a
        Slice) into a BoundingRegion specification.

        Exact inverse of _boundsspec2slicespec().
        """
        r1,r2,c1,c2 = slicespec

        left,bottom = scs.matrix2sheet(r2,c1)
        right, top  = scs.matrix2sheet(r1,c2)

        return ((left,bottom),(right,top))