diff --git a/include/QF/set.h b/include/QF/set.h
index 1261bdb40..52b1fae85 100644
--- a/include/QF/set.h
+++ b/include/QF/set.h
@@ -356,6 +356,7 @@ set_iter_t *set_first_r (set_pool_t *set_pool, const set_t *set);
 set_iter_t *set_next (set_iter_t *set_iter);
 set_iter_t *set_next_r (set_pool_t *set_pool, set_iter_t *set_iter);
 
+struct dstring_s;
 /** Return a human-readable string representing the set.
 
 	Empty sets will be represented by the string "{}". Sets of everything
@@ -369,6 +370,21 @@ set_iter_t *set_next_r (set_pool_t *set_pool, set_iter_t *set_iter);
 				will overwrite the results of preceeding calls.
 */
 const char *set_as_string (const set_t *set);
+/** Return a human-readable string representing the set.
+
+	Empty sets will be represented by the string "{}". Sets of everything
+	will be represented by the string "{...}". Inverted sets will have
+	the first implicit member followed by "..." (eg, "256 ...").
+
+	\param str		dstring to which the representation will be written
+	\param set		The set to be converted to a string.
+	\return			The string held in str
+
+	\warning	The string is NOT cleared, but rather the set representation
+				is appeneded to the string. This makes it more useful when
+				constructing strings in a threaded environment.
+*/
+const char *set_as_string_r (struct dstring_s *str, const set_t *set);
 
 ///@}
 #endif//__QF_set_h
diff --git a/libs/util/set.c b/libs/util/set.c
index dfdd55b4e..6b15154e8 100644
--- a/libs/util/set.c
+++ b/libs/util/set.c
@@ -608,22 +608,19 @@ set_next (set_iter_t *set_iter)
 }
 
 const char *
-set_as_string (const set_t *set)
+set_as_string_r (dstring_t *str, const set_t *set)
 {
-	static dstring_t *str;
 	unsigned    i;
 
-	if (!str)
-		str = dstring_new ();
 	if (set_is_empty (set)) {
-		dstring_copystr (str, "{}");
+		dstring_appendstr (str, "{}");
 		return str->str;
 	}
 	if (set_is_everything (set)) {
-		dstring_copystr (str, "{...}");
+		dstring_appendstr (str, "{...}");
 		return str->str;
 	}
-	dstring_copystr (str, "{");
+	dstring_appendstr (str, "{");
 	for (i = 0; i < set->size; i++) {
 		if (set_is_member (set, i)) {
 			if (str->str[1])
@@ -632,8 +629,21 @@ set_as_string (const set_t *set)
 				dasprintf (str, "%d", i);
 		}
 	}
-	if (set->inverted)
+	if (set->inverted) {
 		dasprintf (str, "%s%d ...", str->str[1] ? " " : "", i);
+	}
 	dstring_appendstr (str, "}");
 	return str->str;
 }
+
+const char *
+set_as_string (const set_t *set)
+{
+	static dstring_t *str;
+
+	if (!str) {
+		str = dstring_new ();
+	}
+	dstring_clearstr (str);
+	return set_as_string_r (str, set);
+}