== と = が並んで読みづらい・・・。 var strings: opaque Collection where _.Element == String = ["hello", "world"]let strings: Any<Sequence where .Iterator.Element == String> = ["a", "b", "c"] だから、書き換えたら↓みたいな感じ? let strings: Sequence where _.Iterator.Element == String = ["a", "b", "c"]opaque がなければ Generalized existential になるのか。