这句话该怎么翻译？

["Vern Sun" <s5unty@gmail.com>]

绝对和debian有关 :)
> "If you'd like to include a given package only if it's available, and not fail otherwise, put a question mark at the end of the line."

"..only if it's available, and not fail otherwise..." <-- 看不懂
请帮忙，谢谢！

-- 
Vern
http://www.duodes.com/