Create a function that takes two lists of numbers sorted in ascending order and returns a list of numbers which are common to both the input lists.
common_elements([-1, 3, 4, 6, 7, 9], [1, 3]) ➞ [3]
common_elements([1, 3, 4, 6, 7, 9], [1, 2, 3, 4, 7, 10]) ➞ [1, 3, 4, 7]
common_elements([1, 2, 2, 2, 3, 4, 5], [1, 2, 4, 5]) ➞ [1, 2, 4, 5]
common_elements([1, 2, 3, 4, 5], [10, 12, 13, 15]) ➞ []