keymap.c

uint16_t last_keycode = _______;
bool process_record_user(uint16_t keycode, keyrecord_t *record) {
	switch (keycode) {
		case KC_COMM:
			if (last_keycode == KC_DOT) {
				if (record -> event.pressed) {
					register_code(KC_SLSH);
				} else {
					unregister_code(KC_SLSH);
				}

				last_keycode = KC_SLSH;

				return false;
			}

		default:
			last_keycode = keycode;
			return true;
	}
}