input: Write a function that calculates the Fibonacci sequence up to the nth position. If n is 0, return 0. If n is 1, return 1. Otherwise, return the sum of the two preceding numbers in the sequence.
  
  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