[Haskell-cafe] what does ' mean?

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