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 other positions, return the sum of the Fibonacci numbers at the two preceding positions.
  def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)
  
  assert fibonacci(10)==55
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(5)==5
  assert fibonacci(7)==13
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)