Related to Stack Overflow question http://stackoverflow.com/questions/37447231/misuse-of-guard-statement-to-replace-nil-checking

LinkedList.swift

func subscript(index: Int) -> T {
    guard index >= 0 && index < count else {
        fatalError("Index out of range 0 ..< \(count)")
    }
    
    switch index {
    case 0:
        return head!.value
    case count-1:
        return tail!.value
    default:
        var currentNode = head!
        for _ in 0 ..< index {
            currentNode = currentNode.next!
        }
        return currentNode.value
    }
}