diff --git a/ev.c b/ev.c
index 545b88e..cd542ca 100644
--- a/ev.c
+++ b/ev.c
@@ -567,6 +567,9 @@ ev_sleep (ev_tstamp delay)
       tv.tv_sec  = (time_t)delay;
       tv.tv_usec = (long)((delay - (ev_tstamp)(tv.tv_sec)) * 1e6);
 
+      /* here we rely on sys/time.h + sys/types.h + unistd.h providing select */
+      /* somehting nto guaranteed by newer posix versions, but guaranteed */
+      /* by older ones */
       select (0, 0, 0, 0, &tv);
 #endif
     }