def Context<ROLE>.ComputeNonce(seq):
  seq_bytes = I2OSP(seq, Nn)
  return xor(self.base_nonce, seq_bytes)

def Context<ROLE>.IncrementSeq():
  if self.seq >= (1 << (8*Nn)) - 1:
    raise MessageLimitReachedError
  self.seq += 1