input: Write a python function to compute the sum of two numbers using recursion.
  assert recursive_sum(0, 5) == 5
  assert recursive_sum(3, 4) == 7
  assert recursive_sum(10, 2) == 12
  assert recursive_sum(5, 7) == 12
output: def recursive_sum(a, b):
      if b == 0:
          return a
      return recursive_sum(a + 1, b - 1)