diff --git a/cameo/zstack.pl b/cameo/zstack.pl
index e5c1d2e..bd5d27f 100755
--- a/cameo/zstack.pl
+++ b/cameo/zstack.pl
@@ -14,7 +14,7 @@
 
 sub usage
 {
-	print STDERR "usage: $0 z-start z-maxstep [file ...]\n";
+	print STDERR "usage: $0 [-r] z-start z-maxstep [file ...]\n";
 	exit(1);
 }
 
@@ -35,6 +35,11 @@ sub issue
 }
 
 
+if ($ARGV[0] eq "-r") {
+	shift @ARGV;
+	$reverse = 1;
+}
+
 $z0 = shift @ARGV;
 $zs = shift @ARGV;
 
@@ -50,9 +55,18 @@ while (<>) {
 	}
 	$z = $3 if /^-?\d+(\.\d*)?\s+-?\d+(\.\d*)?\s+(-?\d+(\.\d*)?)/;
 	$s .= $_;
+	$zmax = $z if $z > $zmax || !defined $zmax;
 }
 $z{$z} .= "$s\n";
 
+if ($reverse) {
+	for (keys %z) {
+		($t{$zmax-$_} = $z{$_}) =~
+		    s/^(-?\d+(\.\d*)?)\s+(-?\d+(\.\d*)?)\s+/$3 $1 /mg;
+	}
+	%z = %t;
+}
+
 $z = $z0;
 for $c (sort { $b <=> $a } keys %z) {
 	while ($z > $c+$zs) {