doc: missing description of a group-reduce param.

* txr.1: Describe the filter-fun optional argument of
group-reduce.

1 files changed, 16 insertions, 2 deletions

diff --git a/txr.1 b/txr.1
index cf7de0be..9d1422ea 100644
--- a/txr.1
+++ b/txr.1
@@ -36795,11 +36795,25 @@ 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
 is invoked on an empty hash table, its net result closely resembles a