adding get_tag definition

include/boost/mpl/get_tag.hpp

@@ -0,0 +1,26 @@
+
+#ifndef BOOST_MPL_GET_TAG_HPP_INCLUDED
+#define BOOST_MPL_GET_TAG_HPP_INCLUDED
+
+// Copyright Sergey Krivonos 2017
+//
+// 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)
+//
+// See http://www.boost.org/libs/mpl for documentation.
+
+// $Id$
+// $Date$
+// $Revision$
+
+
+#define BOOST_MPL_GET_TAG_DEF(TAG) \
+    namespace boost { namespace mpl { \
+        template<typename T> \
+        struct get_##TAG { \
+            typedef typename T::TAG type; \
+        }; \
+    }}
+
+#endif