input: 
  def fibonacci_seq(n):
      """Generate the nth number in the Fibonacci sequence.
      >>> fibonacci_seq(0)
      0
      >>> fibonacci_seq(1)
      1
      >>> fibonacci_seq(5)
      5
      >>> fibonacci_seq(10)
      55
      """
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci_seq(n - 1) + fibonacci_seq(n - 2)
  
output:     if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b