: open(dbname[, mode])
    dbnameǻꤷǡ١⡼ɤ
    modeꤷƥץ󤷤ޤmodeξ
    άͤ0666ǤmodeȤnil
    ȥǡ١¸ߤʤˤϿʥǡ١餺
    nil֤ޤ

