@Override
    public void addField(Field<?> field)
    {
        if (fields.put(field.name, field) != null)
            throw err("Duplicate extension field: " + field.name, 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