def expiration_maintenance(self):
if self.cellarray[self.refresh_head] != <NUM_LIT:0>:<EOL><INDENT>self.cellarray[self.refresh_head] -= <NUM_LIT:1><EOL><DEDENT>self.refresh_head = (self.refresh_head + <NUM_LIT:1>) % self.nbr_bits<EOL>
Decrement cell value if not zero
            This maintenance process need to executed each self.compute_refresh_time()