Seems I can't "unforget" erroneously forgotten files, so will have to forget them and then re-add them
#ifndef BOOST_IS_PLACEHOLDER_HPP_INCLUDED
#define BOOST_IS_PLACEHOLDER_HPP_INCLUDED
// MS compatible compilers support #pragma once
#if defined( _MSC_VER ) && ( _MSC_VER >= 1020 )
# pragma once
#endif
// is_placeholder.hpp - TR1 is_placeholder metafunction
//
// Copyright (c) 2006 Peter Dimov
//
// 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
namespace boost
{
template< class T > struct is_placeholder
{
enum _vt { value = 0 };
};
} // namespace boost
#endif // #ifndef BOOST_IS_PLACEHOLDER_HPP_INCLUDED