What does =*> mean with regards to context free grammars?

I've been reading a couple books/online references about compiler theory, and keep s开发者_Python百科eeing that particular operator coming up every once in a while (as seen here), specifically when the current topic is context free grammars. What does it mean? As well, how does it differ from =>?

Explanations with examples distinguishing => from =*> would be most helpful.

=> means derives in one step while =*> derives in zero or more steps (the reflexive transitive closure of =>).