/*
 *  call-seq:
 *     filter.bits(index_reader) -> bit_vector
 *
 *  Get the bit_vector used by this filter. This method will usually be used
 *  to group filters or apply filters to other filters.
 */
static VALUE
frt_f_get_bits(VALUE self, VALUE rindex_reader)
{
    BitVector *bv;
    IndexReader *ir;
    GET_F();
    Data_Get_Struct(rindex_reader, IndexReader, ir);
    bv = filt_get_bv(f, ir);
    return frt_get_bv(bv);
}