[Haskell-cafe] what does ' mean?

[Logesh Pillay]

Dear list

A v. newbie question.

I found the following code on the web

/-- Andrew Bromage
/
/-- If you're doing permutations, then I suppose you want this too:
//-- (subLists might be a more suggestive name)
/combinations *:: [a] -> [[a]]
*combinations [] = [[]]
combinations (x:xs)
    = combinations xs ++ [ x:xs' | xs' <- combinations xs ]

What does ' (apostrophe) mean?  I see no reference to it in Simon 
Thompson's book.

Thanks

Logesh