   NSCalendarUnit.CalendarUnitYearForWeekOfYear
 ]
 
-public func +(left: NSDateComponents, right: NSDateComponents) -> NSDateComponents {
+public func +(lhs: NSDateComponents, rhs: NSDateComponents) -> NSDateComponents {
   map(calendarUnits) { unit -> () in
-    let leftValue = left.valueForComponent(unit)
-    let rightValue = right.valueForComponent(unit)
+    let leftValue = lhs.valueForComponent(unit)
+    let rightValue = rhs.valueForComponent(unit)
     var value: Int
 
     if leftValue != Int(NSDateComponentUndefined) && rightValue != Int(NSDateComponentUndefined) {

public func <(lhs: NSDate, rhs: NSDate) -> Bool {
  return lhs.compare(rhs) == .OrderedAscending
}
extension NSDate: Comparable { }
      value = leftValue
    }

    lhs.setValue(value, forComponent: unit)
  }
  return lhs
}

public func-(date: NSDate, components: NSDateComponents) -> NSDate {
