Complete the following function that counts the even numbers in a list of integers. For example, if list a contains the elements 3,5,4,−1, and 0, the call count_evens(a) would evaluate to 2, since a...

Complete the following function that counts the even numbers in a list of integers. For example, if list a contains the elements 3,5,4,−1, and 0, the call count_evens(a) would evaluate to 2, since a contains two even numbers: 4 and 0. The function returns zero if the list is empty. The function does not affect the contents of the list.