I think I have some idea on how this was obtained. Just iluminate me onto something. If I have B<A> where B and A are operators and <A> refers to the mean value of A. If I do <B<A>> can this equal <B><A> also, Can <<A><B>> = <A><B> ... ? Thank you

I know I am being very vague with this question, so forgive me if I commited any obvious mathematical flaw.