static int mkv_write_simpletag(AVIOContext *pb, AVDictionaryEntry *t)
{
    uint8_t *key = av_strdup(t->key);
    uint8_t *p   = key;
    const uint8_t *lang = NULL;
    ebml_master tag;

    if (!key)
        return AVERROR(ENOMEM);

    if ((p = strrchr(p, '-')) &&
        (lang = av_convert_lang_to(p + 1, AV_LANG_ISO639_2_BIBL)))
        *p = 0;

    p = key;
    while (*p) {
        if (*p == ' ')
            *p = '_';
        else if (*p >= 'a' && *p <= 'z')
            *p -= 'a' - 'A';
        p++;
    }

    tag = start_ebml_master(pb, MATROSKA_ID_SIMPLETAG, 0);
    put_ebml_string(pb, MATROSKA_ID_TAGNAME, key);
    if (lang)
        put_ebml_string(pb, MATROSKA_ID_TAGLANG, lang);
    put_ebml_string(pb, MATROSKA_ID_TAGSTRING, t->value);
    end_ebml_master(pb, tag);

    av_freep(&key);
    return 0;
}