+/* layer to move during press Fn key */
+uint8_t keymap_fn_layer(uint8_t fn_bits);
+
+/* keycode to send when release Fn key without using */
+uint8_t keymap_fn_keycode(uint8_t fn_bits);
+
+/* whether special key combination */
+bool keymap_is_special_mode(uint8_t fn_bits);