[+ =] key : direct sum to the screen registry.
 [-] key : direct substraction to the screen registry.
 [X] and [:] key : the result is added (with [+]) or substracted (with [-]) to the screen registry.
 [GT] key : Grand Total - All calculations are added or substracted to the screen registry and to the GT registry.  [C] - [AC] : [C] clears only the screen registry and [AC] clears both.
 IR key : number of calculations ([-] or [+] key) since last reset by [AC].