diff --git a/hgmerge b/hgmerge
--- a/hgmerge
+++ b/hgmerge
@@ -4,11 +4,15 @@
 #
 # This tries to find a way to do three-way merge on the current system.
 # The result ought to end up in $1.
+#
+# Environment variables set by Mercurial:
+# HG_ROOT            repo root
+# HG_FILE            name of file within repo
+# HG_MY_NODE         revision being merged
+# HG_OTHER_NODE      revision being merged
 
 set -e # bail out quickly on failure
 
-echo $1 $2 $3
-
 LOCAL="$1"
 BASE="$2"
 OTHER="$3"