* @param identifier The {@code IdentifiedObject} identifier.
* @return {@code this}, for method call chaining.
*/
public B addIdentifier(final String identifier) {
ensureNonNull("identifier", identifier);
identifiers.add(new ImmutableIdentifier((Citation) properties.get(ReferenceIdentifier.AUTHORITY_KEY),
(String) properties.get(ReferenceIdentifier.CODESPACE_KEY), identifier));
return self();
}