treat cases where size > UNIT_SIZE in is_`marked_at`, `mark_at`
treat cases where size > UNIT_SIZE in is_`marked_at`, `mark_at`

koba-e964/picrin,koba-e964/picrin,picrin-scheme/picrin,dcurrie/picrin,omasanori/picrin,dcurrie/picrin,koba-e964/picrin,omasanori/picrin,picrin-scheme/picrin