singleOrElse method
Emits the single element of this Observable, or evaluates the tooFew
callback if there was no element, or evaluates the tooMany
callback if
there was more than 1 element.
Implementation
Observable<T> singleOrElse(
{required Map0<T> tooFew, required Map0<T> tooMany}) =>
SingleObservable<T>(this, tooFew, tooMany);