public protocol Sequence {

    /// A type representing the sequence's elements.
    associatedtype Element where Self.Element == Self.Iterator.Element

    /// A type that provides the sequence's iteration interface and
    /// encapsulates its iteration state.
    associatedtype Iterator : IteratorProtocol

    /// Returns an iterator over the elements of this sequence.
    func makeIterator() -> Self.Iterator

    /// A value less than or equal to the number of elements in the sequence,
    /// calculated nondestructively.
    ///
    /// The default implementation returns 0. If you provide your own
    /// implementation, make sure to compute the value nondestructively.
    ///
    /// - Complexity: O(1), except if the sequence also conforms to `Collection`.
    ///   In this case, see the documentation of `Collection.underestimatedCount`.
    var underestimatedCount: Int { get }

    /// Call `body(p)`, where `p` is a pointer to the collection's
    /// contiguous storage.  If no such storage exists, it is
    /// first created.  If the collection does not support an internal
    /// representation in a form of contiguous storage, `body` is not
    /// called and `nil` is returned.
    ///
    /// A `Collection` that provides its own implementation of this method
    /// must also guarantee that an equivalent buffer of its `SubSequence` 
    /// can be generated by advancing the pointer by the distance to the
    /// slice's `startIndex`.
    func withContiguousStorageIfAvailable<R>(_ body: (UnsafeBufferPointer<Self.Element>) throws -> R) rethrows -> R?
}

Conforming Types AnySequence, Collection, Dictionary, DropFirstSequence, DropWhileSequence, EmptyCollection, EmptyCollection.Iterator, EnumeratedSequence, EnumeratedSequence.Iterator, FlattenSequence, IndexingIterator, IteratorSequence, JoinedSequence, LazyDropWhileSequence, LazyFilterSequence, LazyFilterSequence.Iterator, LazyMapSequence.Iterator, LazyPrefixWhileSequence, LazyPrefixWhileSequence.Iterator, LazySequence, LazySequenceProtocol, PrefixSequence, ReversedCollection, ReversedCollection.Iterator, Set, StrideThrough, StrideTo, UnfoldSequence, UnsafeBufferPointer, UnsafeMutableBufferPointer, UnsafeMutableRawBufferPointer, UnsafeRawBufferPointer, UnsafeRawBufferPointer.Iterator, Zip2Sequence

Associated Types

associatedtype Element
associatedtype Iterator

Sequence 序列 由于 associatedtype Iterator : IteratorProtocol 所以支持迭代器forin
作为数组的基础， 它赋予了数组遍历的能力，从前、后取值以及各种高阶函数的能力。
但是他不具备 「索引」的能力， 无法取出序列中的第N个数值， 只能遍历找到他。
当你进行数据操作时候，你觉得需要正、反向遍历时候， 不妨考虑一下sequence协议。
另外索引的能力在稍后介绍的 collection 协议中。

    @inlinable public func shuffled<T>(using generator: inout T) -> [Self.Element] where T : RandomNumberGenerator
    @inlinable public func shuffled() -> [Self.Element]
    @inlinable public var lazy: LazySequence<Self> { get }

    @available(swift, deprecated: 4.1, renamed: "compactMap(_:)", message: "Please use compactMap(_:) for the case where closure returns an optional value")
    public func flatMap<ElementOfResult>(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]

    @inlinable public func map<T>(_ transform: (Self.Element) throws -> T) rethrows -> [T]

    @inlinable public func filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]

    @inlinable public var underestimatedCount: Int { get }

    @inlinable public func forEach(_ body: (Self.Element) throws -> Void) rethrows

    @inlinable public func first(where predicate: (Self.Element) throws -> Bool) rethrows -> Self.Element?
    // 数组分割
    @inlinable public func split(maxSplits: Int = Int.max, omittingEmptySubsequences: Bool = true, whereSeparator isSeparator: (Self.Element) throws -> Bool) rethrows -> [ArraySlice<Self.Element>]
     // 获取数组的后N个
    @inlinable public func suffix(_ maxLength: Int) -> [Self.Element]
    // 这个与js类似了， 从前面去sequence的N个元素， 返回新的sequence
    @inlinable public func dropFirst(_ k: Int = 1) -> DropFirstSequence<Self>
    // 后面
    @inlinable public func dropLast(_ k: Int = 1) -> [Self.Element]

    @inlinable public func drop(while predicate: (Self.Element) throws -> Bool) rethrows -> DropWhileSequence<Self>
     // 获取sequence前N位
    @inlinable public func prefix(_ maxLength: Int) -> PrefixSequence<Self>

    @inlinable public func prefix(while predicate: (Self.Element) throws -> Bool) rethrows -> [Self.Element]
    // 需要研究
    @inlinable public func withContiguousStorageIfAvailable<R>(_ body: (UnsafeBufferPointer<Self.Element>) throws -> R) rethrows -> R?

    @inlinable public func enumerated() -> EnumeratedSequence<Self>

    @warn_unqualified_access
    @inlinable public func min(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element?

    @warn_unqualified_access
    @inlinable public func max(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Self.Element?
    // 数组是否以某个数组开头
    @inlinable public func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix, by areEquivalent: (Self.Element, PossiblePrefix.Element) throws -> Bool) rethrows -> Bool where PossiblePrefix : Sequence
    // 比较两个数组是否相同
    @inlinable public func elementsEqual<OtherSequence>(_ other: OtherSequence, by areEquivalent: (Self.Element, OtherSequence.Element) throws -> Bool) rethrows -> Bool where OtherSequence : Sequence

    @inlinable public func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence, by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Bool where OtherSequence : Sequence, Self.Element == OtherSequence.Element

    @inlinable public func contains(where predicate: (Self.Element) throws -> Bool) rethrows -> Bool
    // 是否都满足某个条件
    @inlinable public func allSatisfy(_ predicate: (Self.Element) throws -> Bool) rethrows -> Bool

    @inlinable public func reduce<Result>(_ initialResult: Result, _ nextPartialResult: (Result, Self.Element) throws -> Result) rethrows -> Result

    @inlinable public func reduce<Result>(into initialResult: Result, _ updateAccumulatingResult: (inout Result, Self.Element) throws -> ()) rethrows -> Result

    @inlinable public func reversed() -> [Self.Element]

    @inlinable public func flatMap<SegmentOfResult>(_ transform: (Self.Element) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.Element] where SegmentOfResult : Sequence

    @inlinable public func compactMap<ElementOfResult>(_ transform: (Self.Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult]

    @inlinable public func sorted(by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> [Self.Element]

extension Sequence where Self.Element : Sequence {
    @inlinable public func joined() -> FlattenSequence<Self>
    @inlinable public func joined<Separator>(separator: Separator) -> JoinedSequence<Self> where Separator : Sequence, Separator.Element == Self.Element.Element
}