From 44d5dfa528977e84239c98c97e8e8aee30bda46e Mon Sep 17 00:00:00 2001
From: Tony Garnock-Jones <tonygarnockjones@gmail.com>
Date: Sat, 14 Jun 2014 18:56:52 -0400
Subject: [PATCH] Provide an identity for set-union

---
 minimart/gestalt.rkt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/minimart/gestalt.rkt b/minimart/gestalt.rkt
index 9c07f1c..6b7d730 100644
--- a/minimart/gestalt.rkt
+++ b/minimart/gestalt.rkt
@@ -115,7 +115,7 @@
 (define (gestalt-match-value g body metalevel is-feedback?)
   (define extract-matcher (if is-feedback? cdr car)) ;; feedback targets advertisers/publishers
   (define (pids-at level) (matcher-match-value (extract-matcher level) body))
-  (apply set-union (map pids-at (gestalt-metalevel-ref g metalevel))))
+  (apply set-union (set) (map pids-at (gestalt-metalevel-ref g metalevel))))
 
 ;; (Listof Projection) -> CompiledProjection
 ;; For use with gestalt-project.