     log_info(stderr, "    memory usage   : %.1f MB\n", float( qgram_index.used_device_memory() ) / float(1024*1024) );
 }
 
-// build a q-group index
+// build a q-group index from a string
 //
 template <typename string_type>
 void test_qgroup_index_build(

    uint32 operator() (const uint2 range) const { return range.y - range.x > 0 ? 1u : 0u; }
};

template <typename genome_string>
void build_qgrams(
    const uint32                    Q,


    // test q-gram index
    {
        log_visible(stderr, "  testing q-gram index... started\n");

        QGramIndexDevice qgram_index;

        test_qgram_index_build(
