| Copyright (C) 2011 - 2012 Andrzej Krzemienski. | |
| Use, modification, and distribution is subject to the Boost Software | |
| License, Version 1.0. (See accompanying file LICENSE_1_0.txt or 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 |