Convert an iterable boundsspec (supplying l,b,r,t of a
        BoundingRegion) into a Slice specification.

        Includes all units whose centers are within the specified
        sheet-coordinate bounds specified by boundsspec.

        Exact inverse of _slicespec2boundsspec().

def _boundsspec2slicespec(boundsspec,scs):
        """
        Convert an iterable boundsspec (supplying l,b,r,t of a
        BoundingRegion) into a Slice specification.

        Includes all units whose centers are within the specified
        sheet-coordinate bounds specified by boundsspec.

        Exact inverse of _slicespec2boundsspec().
        """
        l,b,r,t = boundsspec

        t_m,l_m = scs.sheet2matrix(l,t)
        b_m,r_m = scs.sheet2matrix(r,b)

        l_idx = int(np.ceil(l_m-0.5))
        t_idx = int(np.ceil(t_m-0.5))
        # CBENHANCEMENT: Python 2.6's math.trunc()?
        r_idx = int(np.floor(r_m+0.5))
        b_idx = int(np.floor(b_m+0.5))

        return t_idx,b_idx,l_idx,r_idx