ko.observable.fn.virtual = function (override, opts) {
    /// <summary>
    /// Allow a local observable to be overridden by another observable.
    /// </summary>
    /// <param name="override" type="Object">
    /// The overriding observable, or if the "name" option is present, an object which contains the
    /// overriding observable.
    /// </param>
    /// <param name="opts" type="Object">
    /// Options:
    /// - useLocalValue (default: false):
    ///   If true, then the current value of the local observable being overridden is bound to the
    ///   overriding observable before being returned.
    /// - passthrough (default: false):
    ///   If true, then the local observable is not overridden. Instead, any change to the local
    ///   observable is passed along to the overriding observable.
    /// - name (default: null):
    ///   If the "override" parameter is not the overriding observable, but is actually an object
    ///   which contains the overriding observable, then the "name" option represents the property
    ///   name of the overriding observable within the "override" object
    ///   (e.g. ko.unwrap(override)[opts.name] would be the overriding observable).
    /// </param>
    /// <returns type="Object">
    /// An observable.
    /// </returns>
    var self = this,
        value = this(),
        resultObservable = null,
        overridingObservable = null,
        options = (opts !== null && typeof opts === 'object') ? opts : {},
        settings = {
            useLocalValue: (!!options.useLocalValue),
            passthrough: (!!options.passthrough),
            name: (!!options.name) ? options.name : null
        };
    if (override && (typeof override === 'object' || ko.isObservable(override))) {
        if (settings.name) {
            var overrideContainer = ko.unwrap(override);
            overridingObservable = ko.isObservable(overrideContainer[settings.name]) ? overrideContainer[settings.name] : null;
        } else {
            overridingObservable = ko.isObservable(override) ? override : null;
        }
    }
    if (ko.isObservable(overridingObservable)) {
        if (settings.useLocalValue) {
            overridingObservable(value);
        }
        if (settings.passthrough) {
            this.passthrough = overridingObservable;
            this.subscribe(function (newValue) {
                self.passthrough(newValue);
            });
            resultObservable = this;
        } else {
            resultObservable = overridingObservable;
        }
    } else {
        resultObservable = this;
    }
    return resultObservable;
};