input: Write a function to implement the Fibonacci sequence. The function should return the nth Fibonacci number. If n is 0 or 1, it should return 0 or 1 respectively. For other values of n, it should return the sum of the two preceding numbers in the sequence.
  def fibonacci_seq(n):
      if n==0:
          return 0
      elif n==1:
          return 1
      else:
          return fibonacci_seq(n-1)+fibonacci_seq(n-2)
  assert fibonacci_seq(0)==0
  assert fibonacci_seq(1)==1
  assert fibonacci_seq(2)==1
  assert fibonacci_seq(3)==2
  assert fibonacci_seq(4)==3
output: def fibonacci_seq(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          return fibonacci_seq(n-1) + fibonacci_seq(n-2)