swift- mutating

struct Stack<Element> {

var items = [Element]()

func push(_ item:Element){

self.items.append(item)

}

mutating func pop() -> Element{

return self.items.removeLast()

}

}

Cannot use mutating member on immutable value: 'self' is immutable

Mark method 'mutating' to make 'self' mutable

protocol Response {

var _metrics: AnyObject? { get set }

func add(_ metrics: AnyObject?)

}

extension Response {

func add(_ metrics: AnyObject?) {

_metrics = metrics

}

}

Cannot assign to property: 'self' is immutable

Mark method 'mutating' to make 'self' mutable