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class Ring:
   
def __init__(self, l):
       
if not len(l):
           
raise "ring must have at least one element"
       
self._data = l

   
def __repr__(self):
       
return repr(self._data)

   
def __len__(self):
       
return len(self._data)

   
def __getitem__(self, i):
       
return self._data[i]

   
def turn(self):
       
last = self._data.pop(-1)
       
self._data.insert(0, last)

   
def first(self):
       
return self._data[0]

   
def last(self):
       
return self._data[-1]

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