input: Write a function to implement the Fibonacci sequence. The function should take an integer `n` as input and return the nth number in the Fibonacci sequence. If `n` is 0, return 0. If `n` is 1, return 1. For any other value of `n`, return the sum of the (n-1)th and (n-2)th numbers in the Fibonacci sequence.
  assert fibonacci_seq(0)==0
  assert fibonacci_seq(1)==1
  assert fibonacci_seq(5)==5
  assert fibonacci_seq(10)==55
  assert fibonacci_seq(15)==610
output: def fibonacci_seq(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b