Stack.swift

struct Stack<T>: CustomStringConvertible {
    init(_ s:_Stack<T>)       { stack = s                }
    init(             )       { self.init(createStack()) }
    func push(_ el: T )       { stack.push(el)           }
    func pop (        ) -> T? { stack.pop()              }
    func peek(        ) -> T? { stack.peek()             }
    var description: String   { stack.string()           }
    private let stack: _Stack<T>
}