input: Write a function to implement the Fibonacci sequence. The function should take an integer `n` as input and return the nth Fibonacci number. The Fibonacci sequence is defined as F(0) = 0, F(1) = 1, and F(n) = F(n-1) + F(n-2) for n >= 2.
  assert fibonacci_seq(0)==0
  assert fibonacci_seq(1)==1
  assert fibonacci_seq(2)==1
  assert fibonacci_seq(5)==5
  assert fibonacci_seq(10)==55
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