2> let b: ArraySlice<String> = [""] b: ArraySlice<String> = 1 value { [0] = "" } 3> b.prefix(3).map { $0 } error: repl.swift:3:1: error: ambiguous use of 'prefix' b.prefix(3).map { $0 } ^ 3> b.prefix(3) $R0: ArraySlice<String> = 1 value { [0] = "" } 2> let b: ArraySlice<String> = [""] b: ArraySlice<String> = 1 value { [0] = "" } 3> b.prefix(3).map { $0 } error: repl.swift:3:1: error: ambiguous use of 'prefix' b.prefix(3).map { $0 } ^ 3> b.prefix(3) $R0: ArraySlice<String> = 1 value { [0] = "" }SubSequence == Sequence じゃないのかextension Sequence where SubSequence : Sequence, SubSequence.Iterator.Element == Iterator.Element, SubSequence.SubSequence == SubSequence { public func prefix(_ maxLength: Int) -> AnySequence<Iterator.Element> { } }