-----Description-----  
This task involves determining whether a specified key appears exactly once in an array. The goal is to verify the uniqueness of the key's occurrence without prescribing any specific approach or implementation method.

-----Input-----  
The input consists of:  
• a: An array of integers.  
• key: An integer representing the element whose occurrence is to be checked.

-----Output-----  
The output is a Boolean value that:  
• Is true if the key appears exactly once in the array.  
• Is false otherwise.

-----Note-----  
The function should correctly handle arrays with no occurrences of the key, multiple occurrences, and exactly one occurrence.