* If the expression starts with a digit or '-', it's an integer: return it. * If the expression starts with a letter, it's a variable. Recall it by checking the current scope in reverse order. * Otherwise, group the tokens (variables or expressions) within this expression by counting the "balance" `bal` of the occurrences of `'('` minus the number of occurrences of `')'`. When the balance is zero, we have ended a token. For example, `(add 1 (add 2 3))` should have tokens `'1'` and `'(add 2 3)'`. * For add and mult expressions, evaluate each token and return the addition or multiplication of them. * For let expressions, evaluate each expression sequentially and assign it to the variable in the current scope, then return the evaluation of the final expression.