input: Write a function to calculate the nth Fibonacci number. If the input is 0, return 0. If the input is 1, return 1. For any other input, 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(2) == 1
output: def fibonacci(n):
   if n == 0:
    return 0
   elif n == 1:
    return 1
   else:
    return fibonacci(n - 1) + fibonacci(n - 2)