Achieving it is not only the chief means of adding to your regular duties.
An alibi is a reason with a bad reputation.
Some of the world's greatest feats were accomplished by people not smart enough to know they were impossible.
Accomplishing the impossible means only the boss will add it to your regular duties.
Autumn is a season followed immediately by looking forward to spring.
In debating the respective merits of dogs and cats, not having to walk a cat when it's 20 below zero deserves consideration.