From Andrey Mirtchovski - The explanation is in the 9fans mail list, the
subject is "APE exit()"

Basically it resolves the issue of a suicide that kills too far back,
including the parent shell.

Reference: /n/sources/patch/applied/apelib_kill
Date: Sat Mar  3 14:49:00 CET 2007
Signed-off-by: lucio@proxima.alt.za

--- /sys/src/ape/lib/ap/plan9/kill.c	Sat Mar  3 14:40:50 2007
+++ /sys/src/ape/lib/ap/plan9/kill.c	Sat Mar  3 14:40:41 2007
@@ -46,9 +46,12 @@
 	if(pid < 0) {
 		sid = getpgrp();
 		mpid = getpid();
-		setpgid(mpid, -pid);
-		r = note(mpid, msg, "/proc/%d/notepg");
-		setpgid(mpid, sid);
+		if(setpgid(mpid, -pid) > 0) {
+			r = note(mpid, msg, "/proc/%d/notepg");
+			setpgid(mpid, sid);
+		} else {
+			r = -1;
+		}
 	} else if(pid == 0)
 		r = note(getpid(), msg, "/proc/%d/notepg");
 	else