Notion of unused parameter in 0-1-11 and 0-1-12

[misra cpp]

'a' is used – see the definition of “used” in the Glossary of MISRA C++:2008:

An object is used if it is:

• The subject of a cast;
  
• Explicitly initialized at declaration time;
  
• An operand in an expression;
  
• Referenced.

In this case, 'a' is used as an operand in an expression.