input: Write a python function to multiply two numbers using recursion.
  assert multiply(3, 4) == 12
  assert multiply(5, 0) == 0
  assert multiply(7, 6) == 42
output: def multiply(a, b):
      if b == 0:
          return 0
      elif b > 0:
          return a + multiply(a, b - 1)
      else:
          return -multiply(a, -b)