title: knockout mapping with computed fields
body: <p>I'm getting data from a WCF service,and i map, and bind the data with my DOM object :</p>
  
  <pre><code>var PayinyVM = {};
  
      $.getJSON('/service/PaidService.svc/PaidList', function (data) {
          var tmp = JSON.stringify(data.d);
  
          PayinyVM.model = ko.mapping.fromJSON(tmp);
          ko.applyBindings(PayinyVM);
      }); 
  </code></pre>
  
  <p>the result is shown as excepted on my DOM bind it to model. What I couldn't find out is how to add some computed observable let's say my data is returning people with FirstName and LastName, how can I make a computed observable FullName with FN + ' ' + LN. </p>