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