[Haskell-cafe] simple Haskell question - List comprehensions

Hi, I'm just working through Hutton's "Programming in Haskell" and I found an exercise which I can't solve, although it looks simple. Maybe someone here could give me a hint? Exercise: Show how the single comprehension [(x,y) | x <- [1,2,3], y <- [4,5,6]] with two generators can be re-expressed using two comprehensions with single generators. Hint: make use of the library function _concat_. Thank you very much, Philip