[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    Copyright (C) 2001-2005 Dan Marsden
    Copyright (C) 2001-2010 Thomas Heller

    Distributed under 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)
===============================================================================/]


[section Placeholder Unification]

Phoenix uses `boost::is_placeholder` for recognizing placeholders:

    template <typename T>
    struct is_placeholder
    {
        static const int value = 0;
    };

To adapt your own placeholder, the nested value needs to be greater than 0
for your types. This is done by specializing this trait.

[endsect]