code: public boolean containsKey(Object key) {
      if(wrappedBindings.containsKey(key)) {
        return true;
      }
      if (key instanceof String) {
        return variableContext.containsVariable((String) key);
      }
      else {
        return false;
      }
    }
docs: Dedicated implementation which does not fall back on the {@link #calculateBindingMap()} for performance reasons