(×) :: [a] -> [b] -> [(a,b)]  -- Read as: the × operator takes a list of elements of
                              -- type `a` and a list of type `b`, and gives back a
                              -- list of tuples of one `a` and one `b` element
xs × ys = [(x,y) | x<-xs, y<-ys]

(⊗) :: [[a]] -> [[a]] -> [[a]]
xs ⊗ ys = [x++y | x<-xs, y<-ys]


main :: IO ()
main = do
   print $ [1,2,3] × "pqr"
   print $ ([1,2] × "pq") × "MN"
   print $ [1,2] × ("pq" × "MN")
   print $ ["1","2","3"] ⊗ ["p","q","r"]
   print $ (["1","2"] ⊗ ["p","q"]) ⊗ ["M","N"]
   print $ ["1","2"] ⊗ (["p","q"] ⊗ ["M","N"])
   print $ map pure [1,2,3] ⊗ map pure [5,6,7]