code: function retrieve(key, callback) {
      var value = model.get(key);
  
      // if we have the value, then trigger the callback immediately
      if (typeof value != 'undefined') {
        return callback(null, value);
      }
  
      // otherwise, wait for the value
      model.once('change:' + key, callback.bind(model, null));
    }
docs: #### retrieve
  
  ```
  retrieve(key, callback)
  ```
  
  Get the value of the specified key and pass the result back through the
  provided `callback` (node error first style).  If the value is already
  available in the local Model, then the callback will be triggered immediately.
  If not, the callback will be triggered once the value has been set in the
  local Model.