add_vx_handler
void add_vx_handler(char *, int, void (*f)(cell_t *));
add_vx_handler("sym-to-value", 0, ieee_sym_to_value);
add_vx_handler("value-to-sym", 0, ieee_value_to_sym);
add_vx_handler("enter-input", 1, polled_give_input);
add_vx_handler("read", 1, polled_read);
add_vx_handler("exit-input", 1, polled_take_input);
add_vx_handler("write", 1, polled_write);
extern void add_vx_handler(char *name, int flag, void (*func)(cell_t *));
add_vx_handler("sync", 1, (void (*)(cell_t *))sync_handler);
extern void add_vx_handler(char *, int, void (*)(cell_t *));