code: func (ks *KeyStore) SetSymmetricKey(key []byte, topic Topic, address PssAddress, addtocache bool) (string, error) {
  	if err := validateAddress(address); err != nil {
  		return "", err
  	}
  	return ks.setSymmetricKey(key, topic, address, addtocache, true)
  }
docs: // Links a peer symmetric key (arbitrary byte sequence) to a topic.
  //
  // This is required for symmetrically encrypted message exchange on the given topic.
  //
  // The key is stored in the whisper backend.
  //
  // If addtocache is set to true, the key will be added to the cache of keys
  // used to attempt symmetric decryption of incoming messages.
  //
  // Returns a string id that can be used to retrieve the key bytes
  // from the whisper backend (see pss.GetSymmetricKey())