code: func checkEncryptReceivers(receivers []BoxPublicKey) error {
  	receiverCount := int64(len(receivers))
  	if receiverCount <= 0 || receiverCount > maxReceiverCount {
  		return ErrBadReceivers
  	}
  
  	// Make sure that each receiver only shows up in the set once.
  	receiverSet := make(map[string]bool)
  
  	// Make sure each key hasn't been used before.
  	for _, receiver := range receivers {
  		kid := receiver.ToKID()
  		kidString := string(kid)
  		if receiverSet[kidString] {
  			return ErrRepeatedKey(kid)
  		}
  		receiverSet[kidString] = true
  	}
  
  	return nil
  }
docs: // checkEncryptReceivers does some sanity checking on the
  // receivers. Check that receivers aren't sent to twice; check that
  // there's at least one receiver and not too many receivers.