def _find_nth_substring(haystack, substring, n):
    """ Finds the position of the n-th occurrence of a substring

    Args:
        haystack (str): Main string to find the occurrences in
        substring (str): Substring to search for in the haystack string
        n (int): The occurrence number of the substring to be located. n > 0

    Returns:
        int_or_None: Position index of the n-th substring occurrence in the
        haystack string if found. None if not found.

    """
    parts = haystack.split(substring, n)

    if n <= 0 or len(parts) <= n:
        return None
    else:
        return len(haystack) - len(parts[-1]) - len(substring)