As other noted you have several options. If you want only mathematical functions. You can import only Darwin.
import Darwin
If you want mathematical functions and other standard classes and functions. You can import Foundation.
import Foundation
If you want everything and also classes for user interface, it depends if your playground is for OS X or iOS.
For OS X, you need import Cocoa.
import Cocoa
For iOS, you need import UIKit.
import UIKit
You can easily discover your playground platform by opening File Inspector (⌥⌘1).
