/* we insert into each other's intersection history because these segments might switch
places and we still want to look them up quickly after they did */
dict_put(&s1->scheduled_crossings, (void*)(ptroff_t)(s2->nr), 0);
dict_put(&s2->scheduled_crossings, (void*)(ptroff_t)(s1->nr), 0);
/* we insert into each other's intersection history because these segments might switch
places and we still want to look them up quickly after they did */
dict_put(&s1->scheduled_crossings, (void*)(ptroff_t)(s2->nr), 0);
dict_put(&s2->scheduled_crossings, (void*)(ptroff_t)(s1->nr), 0);