addingisSameAstotheprotocol.swift

protocol Output {

    func isSameAs(o: Output) -> Bool

}

extension Output where Self: Equatable {

    func isSameAs(o: Output) -> Bool {
        guard let o = o as? Self else { return false }
        return self == o
    }

}

extension String: Output {
    // don't need to do anything; the default implementation picks it up
}

var outputs: [Output] = []
func remove(output: Output) {
    for (i, o) in outputs.enumerate() {
        if o.isSameAs(output) {
            outputs.removeAtIndex(i)
        }
    }
}