input: Write a function to calculate the Fibonacci number at a given position. If the position is 0, return 0. If the position is 1, return 1. For any other position, return the sum of the Fibonacci numbers at the preceding two positions.
  
  def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)
  
  n = 10
  assert fibonacci(n)==55
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)