input: Write a function to calculate the Fibonacci number at a given position. If the input position is 0, return 0. If the input position is 1, return 1. For any other position, 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(0)==0
  assert fibonacci(1)==1
  assert fibonacci(10)==55
  assert fibonacci(7)==13
  assert fibonacci(20)==6765
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)