evaluate expression in reverse polish notation
examples :
> calexpgo 1 2 3 + -
4
> calexpgo 10 15 * ; 15 5 /
150
3
> calexpgo 3 DUP *
9
Values are integer. Operators :
- +: addition,
- -: substraction,
- *: multiplication,
- /: integer division,
- %: modulo,
- dup : duplicate top of stack,
- swap : swap top and second of stack,
- drop : remove top of stack,
- over : add second of stack in top
Opérators are case insensitive