input: def fibonacci_seq(n): 
      """Generate the nth Fibonacci number.
      >>> fibonacci_seq(0)
      0
      >>> fibonacci_seq(1)
      1
      >>> 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:     a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b