def _get_boundary(vs, boundary_string):
    """ Return slice representing boundary

    Parameters
    ----------
    boundary_string
        Identifer for boundary. May take one of the following values:
        SURFACE:       [:, :, -1] only the top layer
        BOTTOM:        bottom_mask as set by veros
        else:          [:, :, :] everything
    """

    if boundary_string == 'SURFACE':
        return tuple([slice(None, None, None), slice(None, None, None), -1])

    if boundary_string == 'BOTTOM':
        return vs.bottom_mask

    return tuple([slice(None, None, None)] * 3)