input: Write a function to generate a Python program that calculates 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 two previous positions.
  def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)
  
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(20) == 6765
  assert fibonacci(5) == 5
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)