Problem 8.1: Derive Sum (optional, 0 pts)

Implement derive-sum, a procedure that differentiates a sum by summing the derivatives of the first-operand and second-operand. Use data abstraction for a sum.

Note: the formula for the derivative of a sum is (f(x) + g(x))' = f'(x) + g'(x)

(define (derive-sum expr var)
  'YOUR-CODE-HERE
)

The tests for this section aren't exhaustive, but tests for later parts will fully test it.