Reimplement and document ldict-reduce.

doc/utils.md

@@ -121,6 +121,17 @@ Returns a new dictionary containing only key/value pairs matching the
 given predicate. The procedure ```pred?``` must accept two arguments -
 the key and the value.
 
+    (ldict-reduce init proc ld)
+
+* ```init``` - initial accumulator value
+* ```proc``` - accumulating procedure
+* ```ld``` - a ldict instance
+
+Performs a reduce operation on the pairs of given dictionary using
+```init``` as initial accumulator. The accumulating procedure must
+accept three arguments: the value accumulated so far, the key and the
+value.
+
 ### IO
 
     (import util-io)