问knockout.js - 如何设置一个空的选择？

<select data-bind="options: myOptions, value: myValue"></select> aka
<span data-bind="text: myValue"></span>

<div>
    <button data-bind="click: setMyValue(null)">clear the selection</button>
    <button data-bind="click: setMyValue('one')">select "one"</button>
    <button data-bind="click: setMyValue('four')">select "four"</button>
</div>
<ul data-bind="foreach: log">
    <li>message: <span data-bind="text: message"></span></li>
</ul>

function Model() {
    var self = this;
    self.myOptions = ['one', 'two', 'three', 'four'];
    self.myValue = ko.observable();
    self.setMyValue = function (val) {
        return function(){
            this.log.push({
                message: "ok, trying to set value as " + val
            });
            self.myValue(val);
        };
    };
    self.log = ko.observableArray([]);
}
var model = new Model();
ko.applyBindings(model);