dropUntilOutputFrom() as filtering operator

DropUntilOutputFrom.swift

let on = PassthroughSubject<Void, Never>()
let strings = PassthroughSubject<String, Never>()

strings
    .drop(untilOutputFrom: on)
    .sink(receiveValue: { print($0) })

["a0", "a1", "b2", "a3", "c4", "a5", "d6"]
    .forEach {
        strings.send($0)
        if $0.hasPrefix("b") {
            on.send()
        }
}

// Output:
//    a3
//    c4
//    a5
//    d6