input: Write a function to generate the Fibonacci number at a given position. If the position is 0, return 0. If the position is 1, return 1. For all other positions, 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(20) == 6765
  assert fibonacci(30) == 832040
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)