func flatMap<SegmentOfResult>(_ transform: (Self.Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence
https://developer.apple.com/documentation/swift/sequence/2905332-flatmapfunc flatMap<SegmentOfResult>(_ transform: (Self.Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence
https://developer.apple.com/documentation/swift/sequence/2905332-flatmapArray じゃないといけないのかと思ってた。Array なだけか。