Write a function named index_of that returns the index of a particular value in a list of integers. The function should return the index of the first occurrence of the target value in the list. If the value is not in the list, it should return -1. For example, if a list called list stores the following values:
list = [42, 7, -9, 14, 8, 39, 42, 8, 19, 0]
Then the call index_of(list, 8) should return 4 because the index of the first occurrence of value 8 in the list is at index 4. The call index_of(list, 2) should return -1 because value 2 is not in the list.
