Copyright (C) 2011-2012 Andrzej Krzemienski Distributed under the Boost Software License, Version 1.0 (see accompanying file LICENSE_1_0.txt or a copy at http://www.boost.org/LICENSE_1_0.txt) The idea and interface is based on Boost.Optional library authored by Fernando Luis Cacciola Carballal Home at https://github.com/akrzemi1/Optional