From 266e28cb7129d175ad4af5af5cb45fbbd3054862 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Sun, 5 Mar 2017 10:19:10 -0800
Subject: doc: missing description of a group-reduce param.

* txr.1: Describe the filter-fun optional argument of
group-reduce.
---
 txr.1 | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/txr.1 b/txr.1
index cf7de0be..9d1422ea 100644
--- a/txr.1
+++ b/txr.1
@@ -36795,10 +36795,24 @@ is invoked with two arguments: the accumulator from step 2, and the
 original element from
 .metn seq .
 .IP 4.
-The resulting value is stored back into the hash table under the
-same key.
+The resulting value from step 3 is stored back into the hash table under the
+key from step 2.
 .RE
 
+.IP
+After the above processing, one more step is performed if the
+.meta filter-fun
+argument is present. In this case, the hash table is destructively mapped through
+.meta filter-fun
+before being returned. That is to say, every value in the hash table is
+projected through
+.meta filter-fun
+and stored back in the table under the same key, as if by an invocation the
+.cblk
+.meti (hash-update < hash << filter-fun )
+.cble
+expression.
+
 .IP
 If
 .code group-reduce
-- 
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