#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/list/cat.hpp>
#include <boost/preprocessor/list/fold_left.hpp>
#define LISTS /* ............. */ \
((a, (b, BOOST_PP_NIL)), \
((d, (e, BOOST_PP_NIL)), \
((e, (f, BOOST_PP_NIL)), \
BOOST_PP_NIL))) \
/**/
#define OP(d, state, x) BOOST_PP_CAT(state, BOOST_PP_LIST_CAT_D(d, x))
BOOST_PP_LIST_FOLD_LEFT(OP, _, LISTS) // expands to _abcdef