系统功能语言学（2 ） Context and text are closely related. We can predict the text from context, and prediction the context from the text. Hallidays example: If you come across once upon a time, then you know immediately that you are being told a traditional story, probably a childrens story. There is no other context in which that expression is used. You can therefore predict quite a lot not only about what is going to follow, but also about the situation in which that is actually being used; typically,
