ElementOf.swift

// Reusing the idea in https://gist.github.com/akosma/07249e6718ace1ba6066

precedencegroup ElementOfPrecedence {
    associativity: left
    higherThan: BitwiseShiftPrecedence
}

infix operator ∈ : ElementOfPrecedence
func ∈<T> (x: T, array: [T]) -> Bool where T: Equatable {
    array.contains(x)
}

infix operator ∉ : ElementOfPrecedence
func ∉<T> (x: T, array: [T]) -> Bool where T: Equatable {
    !(x ∈ array)
}

///

assert("alpha" ∈ ["alpha", "beta", "gamma"])
assert("alpha" ∉ ["omega", "gamma", "delta"])