#include <boost/preprocessor/arithmetic/add.hpp>
#include <boost/preprocessor/list/fold_left.hpp>
#include <boost/preprocessor/list/size.hpp>
#define L1 (a, (b, (c, BOOST_PP_NIL)))
#define L2 (x, (y, BOOST_PP_NIL))
#define L3 (p, (q, BOOST_PP_NIL))
#define LIST (L1, (L2, (L3, BOOST_PP_NIL)))
#define OP(d, state, x) /* ... */ \
BOOST_PP_ADD_D( \
d, state, \
BOOST_PP_LIST_SIZE_D(d, x) \
) \
/**/
BOOST_PP_LIST_FOLD_LEFT(OP, 0, LIST) // expands to 7