title: idiom for comparing knockout observables
body: <p>I have the following problem.
  I want to check that the item clicked on in table is not the same as model.selected.</p>
  
  <pre><code>var model= {
  items:  ko.observableArray(),
  selected : ko.observable()
  };
  
  &lt;tbody&gt;
  &lt;!-- ko foreach: model.items --&gt;
  &lt;tr data-bind="click:$parent.model.set_selected_item"&gt;
  &lt;td style="cursor:pointer" data-bind="varchar : title"&gt;&lt;/td&gt;
  &lt;/tr&gt;
  &lt;!-- /ko --&gt;
  &lt;/tbody&gt;
  
  //ID is an observable
  //selected may not be set yet - i.e an empty observable;
  
  
  
   var set_selected_item = function(item){
      //if item is different set 
        model.LandItem_selected(item);
        do_routine(item)
      //else
        //do nothing
      }
  </code></pre>
  
  <p>because the item is an observable is is never null;
  how would I check if the observable has not been set yet?</p>
  
  <p>Any help much appreciated.</p>