input: Write a function to calculate the nth term of the Fibonacci sequence. If n is 0 return 0, if n is 1 return 1, otherwise return the sum of the two preceding terms.
  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