func firsts<S1: Sequence, S2: Sequence>(of sequences: (S1, S2)) -> (S1.Element?, S2.Element?) { sequences.map { $0.first } }
は func firsts<S1: Sequence, S2: Sequence>(of sequences: (S1, S2)) -> (S1.Element?, S2.Element?) { (sequences.0.first, sequences.1.first) }
のシュガーみたいに解釈できると思うんよね。