[Haskell-cafe] what does ' mean?

[yin]

Logesh Pillay wrote:

> 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.

' nothing. xs' does the same as xs_, or _xs, or xs2, but it's readable.