 
 infix operator ∈ { associativity left }
 func ∈<T: Equatable> (left: T, right: [T]) -> Bool {
-    return contains(right, left)
+    return right.contains(left)
 }
 
 func ∈<T> (left: T, right: Set<T>) -> Bool {


prefix operator ∑ {}
prefix func ∑ (values: [Double]) -> Double {
    return reduce(values, 0.0, +)
}

// MARK: Cartesian Product

prefix operator ∏ {}
prefix func ∏ (values: [Double]) -> Double {
    return reduce(values, 1.0, *)
}

// MARK: - Vectors -

// MARK: Nth Derivative

infix operator ′ { associativity left }
func ′(left: (Double -> Double), right: UInt) -> (Double) -> (Double) {
    return (0 ..< right).reduce(left) { (function, _) in
        return function′
    }
}
