int wng_open(struct writer_node_group *g);
void wng_close(struct writer_node_group *g);
struct writer_node_group *wng_new(unsigned num_writers);
int wng_open(struct writer_node_group *g);
void wng_close(struct writer_node_group *g);
struct writer_node_group *wng_new(unsigned num_writers);