Chapter 5: List comprehensions

@laurocaetano and @carlosgaldino Have good times reading the chapter and doing the exercises. Exercise 5 was quite challenge but I gave up, not sure if it is a proper answer for that :disappointed: 

Great stuff seeing LINQ relation to list comprehensions on Erik Meijer lecture.
yeah, that's totally valid, however I did something like: 

````haskell
[[(x, y) | x <- [1, 2, 3]] | y <- [4, 5, 6]]
-- which means -> [[(1,4),(2,4),(3,4)],[(1,5),(2,5),(3,5)],[(1,6),(2,6),(3,6)]]
-- got it?
-- and then, just:
concat [[(1,4),(2,4),(3,4)],[(1,5),(2,5),(3,5)],[(1,6),(2,6),(3,6)]]
-- [(1,4),(2,4),(3,4),(1,5),(2,5),(3,5),(1,6),(2,6),(3,6)]
```