Exercise: flatten a matrix

Write a function flatten(matrix). It returns a new list of all the elements of the nested list matrix: first the elements of the first row, then the second, and so on, in their original order.

• flatten([[1, 2], [3, 4]]) → [1, 2, 3, 4]
• flatten([[5], [], [6, 7]]) → [5, 6, 7]
• flatten([]) → []

Constraints: matrix is a list of lists of numbers; rows may have different lengths and may be empty; an empty matrix gives an empty list; the function does not change matrix itself.

Do not add extra imports unless the task says so explicitly.