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I have this code for a game I am learning how to make:

import SpriteKit
class GameScene: SKScene {
    override func didMoveToView(view: SKView) {
    backgroundColor = SKColor.whiteColor()
    let background = SKSpriteNode(imageNamed: "background1")
    background.position = CGPoint(x: size.width/2, y: size.height/2)
    background.anchorPoint = CGPoint(x: 0.5, y: 0.5) // default
    // background.zRotation = CGFloat(M_PI) / 8
    background.zPosition = -1

    let zombie = SKSpriteNode(imageNamed: "zombie1")
    zombie.position = CGPoint(x: 400, y: 400)
    override func update(currentTime: NSTimeInterval) {
        zombie.position = CGPoint(x: zombie.position.x + 4,
        y: zombie.position.y)

When I put in the override func update in, i get an error at the line where I use the zombie.position. The error says: "use of unresolved indentifier 'zombie'. Why does it do this?