/in/p2.cpp: In function 'opaque_list* traversal(tree_t)':
/in/p2.cpp:178:82: error: no matching function for call to 'list_make(int)'
         return append(append(traversal(tree_left(tree)), list_make(tree_elt(tree))), traversal(tree_right(tree)));
                                                                                  ^
In file included from /in/p2.h:4:0,
                 from /in/p2.cpp:1:
/in/driver/recursive.h:37:8: note: candidate: opaque_list* list_make()
 list_t list_make();
        ^~~~~~~~~
/in/driver/recursive.h:37:8: note:   candidate expects 0 arguments, 1 provided
/in/driver/recursive.h:40:8: note: candidate: opaque_list* list_make(int, list_t)
 list_t list_make(int elt, list_t list);
        ^~~~~~~~~
/in/driver/recursive.h:40:8: note:   candidate expects 2 arguments, 1 provided