         - returns: true if self contains all the items
     */
     public func contains <Element: Equatable> (_ items: Element...) -> Bool {
-        return items.all { (item: Element) -> Bool in self.indexOf(item) >= 0 }
+        return items.all { (item: Element) -> Bool in self.indexOf(item)! >= 0 }
     }
 
     /**

            step = n
        }

        if step < 1 { step = 1 } // Less than 1 results in an infinite loop.
        if n < 1    { n = 0 }    // Allow 0 if user wants [[],[],[]] for some reason.
        if n > count { return [[]] }


        }

        // Less than 1 results in an infinite loop.
        if step! < 1 {
            step = 1
        }
        
