根据模型编了一些指令，其中model中一个等式需要对变量a_n求导数，但是显示不出来，这个external_function应该怎么用，在model前面需要怎么设置一下吗，谢谢高手赐教！

y_ntilde(+1)-y_ntilde=y_ttilde(+1)-y_ttilde-infln+inflh-(external_function(diff)('a_n')(+1)-external_function(diff)('a_n'))+(external_function(diff)('a_t')(+1)-external_function(diff)('a_t'));