{- | Transform a list @[l1, l2, ... ln]@ to (in sloppy notation)
     @[[x1, x2, ... xn] | x1 <- l1, x2 <- l2, ... xn <- ln]@
     (this is just the 'sequence' function!) -}
combine :: [[a]] -> [[a]]
combine = sequence
combine :: [[a]] -> [[a]]
combine = sequence
combine = sequence