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from typing import List
def phib(xs: List[int]) -> bool:
"""
Instructions:
1. `pip install crosshair-tool`
2. modify the precondition `pre` to control the length of your fib sequence
3. run `crosshair check phib.py` in your terminal
pre: len(xs) >= 10
post: __return__ != True
"""
if xs[0] != 0:
return False
if len(xs) > 1:
if xs[1] != 1:
return False
for i in range(2,len(xs)):
if xs[i] != xs[i-1] + xs[i-2]:
return False
return True
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