/in/p2.cpp: In function 'opaque_list* traversal(tree_t)':
/in/p2.cpp:258:108: error: too few arguments to function 'opaque_list* append(list_t, list_t)'
     return append(append(traversal(tree_left(tree)), list_make(tree_elt(tree),traversal(tree_right(tree)))));
                                                                                                            ^
/in/p2.cpp:67:8: note: declared here
 list_t append(list_t first, list_t second)
        ^~~~~~