input: Write a function to sum natural numbers up to n using recursion.
  assert sum_natural_numbers(5) == 15
  assert sum_natural_numbers(10) == 55
  assert sum_natural_numbers(1) == 1
  assert sum_natural_numbers(0) == 0
output: def sum_natural_numbers(n):
      if n <= 0:
          return 0
      return n + sum_natural_numbers(n - 1)