--- OLD/pnm/pnmarith.1	Thu Jan  1 00:00:00 1970
+++ NEW/pnm/pnmarith.1	Thu Jan  1 00:00:00 1970
@@ -4,7 +4,7 @@
 pnmarith - perform arithmetic on two portable anymaps
 .SH SYNOPSIS
 .B pnmarith
-.BR -add | -subtract | -multiply | -difference | -minimum | -maximum.
+.BR -add | -subtract | -multiply | -difference | -minimum | -maximum | -average.
 .I pnmfile1 pnmfile2
 .SH DESCRIPTION
 Reads two portable anymaps as input.
@@ -16,6 +16,10 @@
 anymaps, as if maxval was 1.0, black was 0.0, and a linear scale in between.
 Results that fall outside of [0..1) are truncated.
 .PP
+As a convenience when working with large constant areas, if either file
+is 1x1 then the effect is as if it were the same size as the other file,
+with all pixels being identical to the single actual pixel.
+.PP
 The operator
 .I -difference
 calculates the absolute value of
@@ -28,6 +32,7 @@
 .SH AUTHOR
 Copyright (C) 1989, 1991 by Jef Poskanzer.
 Lightly modified by Marcel Wijkstra <wijkstra@fwi.uva.nl>
+and der Mouse <mouse@rodents.montreal.qc.ca>.
 .\" Permission to use, copy, modify, and distribute this software and its
 .\" documentation for any purpose and without fee is hereby granted, provided
 .\" that the above copyright notice appear in all copies and that both that