UnionOperatorPrecedence.swift

precedencegroup UnionOperatorPrecedence {
    associativity: left
}

infix operator ∪: UnionOperatorPrecedence

func ∪ <T:Equatable>(lhs: [T], rhs:[T])->[T] {
    var result = lhs
    for element in rhs {
        if !lhs.contains(element) {
            result.append(element)
        }
    }
    return result
}