add_pre_merge_hook
add_pre_merge_hook(my_id, &pre_merge_hook);
add_pre_merge_hook(my_id, &pre_merge_hook);
add_pre_merge_hook(my_id, &pre_merge_hook);
add_pre_merge_hook(my_id, &pre_merge_hook);
void add_pre_merge_hook(int client_id, void (*hook)(struct sm_state *cur, struct sm_state *other));
add_pre_merge_hook(comparison_id, &pre_merge_hook);
add_pre_merge_hook(my_id, &pre_merge_hook);
add_pre_merge_hook(my_id, &pre_merge_hook);
add_pre_merge_hook(my_id, &pre_merge_hook);
add_pre_merge_hook(my_id, &pre_merge_hook);