If you get to around lecture 9, a classic example I always tell people to start with is a calculator! For instance, here's the SML code for it: ``` datatype e

If you get to around lecture 9, a classic example I always tell people to start with is a calculator! For instance, here's the SML code for it: ``` datatype exp = Num of int | Plus of exp * exp | Minus of exp * exp | Times of exp * exp | Div of exp * exp ``` Implement the function `eval : exp -> int`, which evaluates the expression as best as it can. Assume no division by zero. Extra credit: Can you implement `eval' : exp -> int option`, that returns `SOME n` if the expression evaluates, and `NONE` if it divides by zero?