input: Write a function to calculate the nth Fibonacci number. The function should return 0 if n is 0, and 1 if n is 1. For all other values of n, it should 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(6)==8
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(7)==13
  assert fibonacci(10)==55
output: def fibonacci(n):
   if n == 0:
    return 0
   elif n == 1:
    return 1
   else:
    return fibonacci(n - 1) + fibonacci(n - 2)