

Sequence
A type that provides sequential, iterated access to its elements.
是最基础的协议，可以通过迭代来获取它的元素。
有两个关联类型：
/// A type representing the sequence's elements.associatedtype Element/// A type that provides the sequence's iteration interface and/// encapsulates its iteration state.associatedtype Iterator : IteratorProtocol where Iterator.Element == Element
遵守Sequence 协议，只需要实现makeIterator()方法即可。 在迭代过程中，Sequence的序列是否被破坏，并没有要求。因此，两次迭代同一个Sequence，可能会得到不同的结果。
Collection
A sequence whose elements can be traversed multiple times, nondestructively, and accessed by an indexed subscript.
在sequence基础上，增加了几个性质
- 多次遍历，不会对序列的结构造成影响
- 可以通过下标索引
引入了几个和索引相关的关联类型
associatedtype Index : Comparable
/// A type that represents the indices that are valid for subscripting the
/// collection, in ascending order.
associatedtype Indices : Collection = DefaultIndices<Self>
where Indices.Element == Index, Indices.Index == Index,Indices.SubSequence == Indices
通过下标获取值的方法如下，复杂度是O(1)
subscript(position: Index) -> Element { get }
可以通过下标来遍历，那么必须定义下标的successor
func index(after i: Self.Index) -> Self.Index
BidirectionalCollection
A collection that supports backward as well as forward traversal.
继承了Collection协议，增加了可以反向遍历的功能。
通过一个下标，可以找到上一个下标。
func index(before i: Index) -> Index
这也为和last相关的方法提供了基础
RandomAccessCollection
A collection that supports efficient random-access index traversal.
继承了BidirectionalCollection，因此可以正向/反向遍历。还对遍历的效率做出来要求，
Random-access collections can move indices any distance and measure the distance between indices in O(1) time
MutableCollection
A collection that supports subscript assignment.
继承了collection协议，提供了可以改变元素值的能力。
override subscript(position: Index) -> Element { get set }
这个下标操作，提供了set方法。 对应的，提供了默认的分片方法(O(n))
/// - Complexity: O(*n*), where *n* is the length of the collection.mutating func partition(by belongsInSecondPartition: (Element) throws -> Bool) rethrows -> Index
也提供了交换方法
/// - Complexity: O(1)mutating func swapAt(_ i: Index, _ j: Index)
RangeReplaceableCollection
A collection that supports replacement of an arbitrary subrange of elements with the elements of another collection.
继承了collection，增加了可以某一个子区间内的元素，可以被另一个collection替代的能力。
为collection增加了插入和删除的能力。
mutating func append(_ newElement: __owned Element)mutating func append<S : Sequence>(contentsOf newElements: __owned S)where S.Element == Elementmutating func insert(_ newElement: __owned Element, at i: Index)mutating func insert<S : Collection>(contentsOf newElements: __owned S, at i: Index)where S.Element == Elementmutating func remove(at i: Index) -> Elementmutating func removeSubrange(_ bounds: Range<Index>)
)
