-----Description----- 
This task requires writing a Lean 4 method that finds the last occurrence of a specified element in a sorted array of integers. The method should return the index corresponding to the last occurrence of the element if it is present; if the element is absent, it should return -1. Additionally, the array must remain unchanged after the method is executed.

-----Input-----
The input consists of:
arr: A sorted array of integers in non-decreasing order.
elem: An integer whose last occurrence position is to be determined.

-----Output-----
The output is an integer:
Returns the index of the last occurrence of the specified integer in the array if it exists.
Returns -1 if the integer is not found in the array.

-----Note-----
The input array is assumed to be sorted in non-decreasing order and remains unchanged by the method.