code: function(context, expr, callback, callbackContext) {
      if (typeof context === 'string') {
        callbackContext = callback;
        callback = expr;
        expr = context;
        context = null;
      }
      var observer = this.observations.createObserver(expr, callback, callbackContext);
      if (context) {
        observer.bind(context, true);
      }
      return observer;
    }
docs: Observes an expression within a given context, calling the callback when it changes and returning the observer.