-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);
-void init_supported_writers(void);
-void *check_writer_arg(const char *wa, int *writer_num);
-struct writer_node_group *setup_default_wng(void);