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def check_index(key):
if not isinstance(key,int): raise TypeError
if key < 0: raise IndexError
class ArithmeticSequence:
def __init__(self,start = 0, step = 1):
self.start = start
self.step = step
self.changed = {}
def __getitem__(self, key):
check_index(key)
try:
return self.changed[key]
except KeyError:
return self.start + key*self.step
def __setitem__(self,key,value):
check_index(key)
self.changed[key] = value