code: func (r *DefaultRecordLayer) DiscardReadKey(epoch Epoch) {
  	if !r.datagram {
  		return
  	}
  
  	_, ok := r.readCiphers[epoch]
  	assert(ok)
  	delete(r.readCiphers, epoch)
  }
docs: // TODO(ekr@rtfm.com): This is never used, which is a bug.