test script:
cat >/tmp/f1 <<'!'
a
c
!
cat >/tmp/f2 <<'!'
b
c
!
fn r {
	echo $*
	$*
}
r comm /tmp/f1 /tmp/f2
r comm -1 /tmp/f1 /tmp/f2
r comm -2 /tmp/f1 /tmp/f2
r comm -3 /tmp/f1 /tmp/f2
r comm -23 /tmp/f1 /tmp/f2
r comm -12 /tmp/f1 /tmp/f2

before:
term% r comm /tmp/f1 /tmp/f2
comm /tmp/f1 /tmp/f2
a
	b
		c
term% r comm -1 /tmp/f1 /tmp/f2
comm -1 /tmp/f1 /tmp/f2
b
		c
term% r comm -2 /tmp/f1 /tmp/f2
comm -2 /tmp/f1 /tmp/f2
a
		c
term% r comm -3 /tmp/f1 /tmp/f2
comm -3 /tmp/f1 /tmp/f2
a
	b
term% r comm -23 /tmp/f1 /tmp/f2
comm -23 /tmp/f1 /tmp/f2
a
term% r comm -12 /tmp/f1 /tmp/f2
comm -12 /tmp/f1 /tmp/f2
	c

note the empty columns in comm -1, comm -2, and comm -12:
there is one tab too many.  it's an easy fix.  i noticed
only later that the plan 9 command was essentially the ancient
unix one, and a diff against that shows that indeed the l = 1 -> l = 2
was a change in going to plan 9, which dented it.

after:

doppio% r comm /tmp/f1 /tmp/f2
comm /tmp/f1 /tmp/f2
a
	b
		c
doppio% r comm -1 /tmp/f1 /tmp/f2
comm -1 /tmp/f1 /tmp/f2
b
	c
doppio% r comm -2 /tmp/f1 /tmp/f2
comm -2 /tmp/f1 /tmp/f2
a
	c
doppio% r comm -3 /tmp/f1 /tmp/f2
comm -3 /tmp/f1 /tmp/f2
a
	b
doppio% r comm -23 /tmp/f1 /tmp/f2
comm -23 /tmp/f1 /tmp/f2
a
doppio% r comm -12 /tmp/f1 /tmp/f2
comm -12 /tmp/f1 /tmp/f2
c

it now correctly suppresses columns.

Reference: /n/sources/patch/applied/commfix
Date: Tue Aug 31 21:22:37 CES 2004

--- /sys/src/cmd/comm.c	Tue Aug 31 21:22:37 2004
+++ /sys/src/cmd/comm.c	Tue Aug 31 21:22:37 2004
@@ -26,7 +26,7 @@
 	ldr[0] = "";
 	ldr[1] = "\t";
 	ldr[2] = "\t\t";
-	l = 2;
+	l = 1;
 	ARGBEGIN{
 	case '1':
 		if(!one) {