along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/
-/* This is a private header, never even think of including this directly! */
+/* This is a private header; never even think of including this directly! */
+
+#if __INCLUDE_LEVEL__ <= 1
+#error "Do not include _sd-common.h directly; it is a private header."
+#endif
#ifndef _sd_printf_
# if __GNUC__ >= 4