input: Write a function to calculate the Fibonacci number at a given position. If the given position is 0 return 0, if it is 1 return 1, else return the sum of the Fibonacci numbers at the previous two 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(15)==610
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci(n-1) + fibonacci(n-2)