// Function with single argument of completion handler
let x: Observable<Void> = .fromCallback(PHPhotoLibrary.shared().performChangesAndWait)
    
// Function with n arguments, the last being a completiong handler
let y: Observable<(Bool, Error?)> = .fromCallback(curry(PHPhotoLibrary.shared().performChanges)({ /* changes */ }))

// Functions that accept completion handlers AND return non-void types aren't compatible
let z: Observable<Void> = .fromCallback(curry(URLSession.shared().dataTask)(URL(string: "")!))

// Because some functions produce multiple callbacks, the Observable returned by `.fromCallback`
// intentionally avoids producing a completion event. To achieve this, the caller may use the 
// `take` operator to produce this event.
let n = Observable<PartialData>.fromCallback(Network.progressiveDownload).take(3)