Rick:
[DONE] Substraction x - y
[DONE] Multiplication x * y
[DONE] Division x / y 
[DONE] Support for parentheses

Johan:
[DONE] Modulo x % y 
[DONE] Exponentiation x ** y 
[DONE] Factorial x! 
[DONE] Extend the calc.l file such that the sequence “PI”
[DONE] Absolute value of an expression