@Override
    public void putExtraOption(String key, Object value)
    {
        if (extraOptions.put(key, value) != null)
            throw err("Duplicate extension option: " + key, getProto());
    }
Represents an extend block declared in either the {@link Proto} or nested in a {@link Message}.

@author Philippe Laflamme
protostuff-parser/src/main/java/io/protostuff/parser/Extension.java