/**
 * Allows publishing and subscribing to series of async values.
 *
 * The `Observable` class is an alias to the `Observable` returned from
 * {@link https://github.com/reactivex/rxjs}. `Observables` are a means of delivering
 * any number of values over any period of time. `Observables` can be thought of as a
 * mixture of `Promise` and `Array`. `Observables` are like `Arrays` in that they can have
 * chained combinators -- like `map`, `reduce`, and `filter` -- attached in order to
 * perform projections and transformations of data. And they are like `Promises`
 * in that they can asynchronously deliver values. But unlike a `Promise`, an
 * `Observable` can emit many values over time, and decides if/when it is completed.
 *
 * `Observable` is also being considered for inclusion in the
 * [ECMAScript spec](https://github.com/zenparsing/es-observable).
 *
 * ## Example
 *
 * A simple example of using an `Observable` is a timer `Observable`, which will
 * notify an `Observer` each time an interval has completed.
 *
 * {@example facade/ts/async/observable.ts region='Observable'}
 *
 * The `Observable` in Angular currently doesn't provide any combinators by default.
 * So it's necessary to explicitly import any combinators that an application requires.
 * There are two ways to import RxJS combinators: pure and patched. The "pure" approach
 * involves importing a combinator as a function every place that an application needs it,
 * then calling the function with the source observable as the context of the function.
 *
 * ## Example
 *
 * {@example facade/ts/async/observable_pure.ts region='Observable'}
 *
 * The "patched" approach to using combinators is to import a special module for
 * each combinator, which will automatically cause the combinator to be patched
 * to the `Observable` prototype, which will make it available to use anywhere in
 * an application after the combinator has been imported once.
 *
 * ## Example
 *
 * (Notice the extra "add" in the path to import `map`)
 *
 * {@example facade/ts/async/observable_patched.ts region='Observable'}
 *
 * Notice that the sequence of operations is now able to be expressed "left-to-right"
 * because `map` is on the `Observable` prototype. For a simple example like this one,
 * the left-to-right expression may seem insignificant. However, when several operators
 * are used in combination, the "callback tree" grows several levels deep, and becomes
 * difficult to read. For this reason, the "patched" approach is the recommended approach
 * to add new operators to `Observable`.
 *
 * For applications that are less sensitive about payload size, the set of core operators
 * can be patched onto the `Observable` prototype with a single import, by importing the
 * `rxjs` module.
 *
 * {@example facade/ts/async/observable_all.ts region='Observable'}
 *
 * Full documentation on RxJS `Observable` and available combinators can be found
 * in the RxJS [Observable docs](http://reactivex.io/RxJS/class/es6/Observable.js~Observable.html).
 *
 */