input: Write a python function to add two numbers recursively using bitwise operations.
  assert add_recursively(1, 2) == 3
  assert add_recursively(10, 20) == 30
  assert add_recursively(0, 0) == 0
  assert add_recursively(15, 5) == 20
output: def add_recursively(a, b):
      if b == 0:
          return a
      else:
          return add_recursively(a ^ b, (a & b) << 1)