Implement ldict-contains? predicate.

doc/utils.md

@@ -49,11 +49,18 @@ populating it with initial data.
 If only one argument is given, the procedure checks whether it is a
 list of equality procedure and acts accordingly.
 
-    (ldict-empty? d)
+    (ldict-empty? ld)
 
-* ```d``` - a ldict instance
+* ```ld``` - a ldict instance
 
-Returns true if given dictionary contains no keys.
+Returns ```#t``` if given dictionary contains no keys.
+
+    (ldict-contains? ld k)
+
+* ```ld``` - a ldict instance
+* ```k``` - a key compatible with given ldict
+
+Returns ```#t``` if given ```ld``` contains given key ```k```.
 
 ### IO