Sentence:  Since N and N ' are both nilpotent and they commute , the operator * * f is nilpotent ; for , using the fact that N and N ' <WSD>commute</WSD> * * f and so when r is sufficiently large every term in this expression for * * f will be 0 . 
Meaning of the word marked with <WSD> tags:  exchange positions without a change in value