input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number at the given position 'n'. If 'n' is 0, return 0. If 'n' is 1, return 1. For any other value of 'n', recursively calculate 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(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(6) == 8
  assert fibonacci(10) == 55
  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)