diff --git a/group-scheduling.rkt b/group-scheduling.rkt
index d47eb52..226596b 100644
--- a/group-scheduling.rkt
+++ b/group-scheduling.rkt
@@ -7,7 +7,7 @@
            math)
 
   ;; echoing verbosity
-  (define verbose? #t)
+  (define verbose? #f)
 
   (define (verbosity b)
     (set! verbose? b)
@@ -19,7 +19,7 @@
       ((_ str ...) (when verbose? (printf str ...)))))
 
   ;; current version
-  (define version "2020-10-11 14:28:50")
+  (define version "2020-10-12 09:48:02")
 
   (define (print-version)
    (printf "version: ~a~n" version))
@@ -28,10 +28,15 @@
   (define %has-met %empty-rel) ;; collection of meetings...
   (define %t0 %empty-rel)      ;; tentative collection...
 
+  ;; meetings recordered via %assert should only be recorded once
+  (use-occurs-check? #t)
+  
   (define (meeting p1 p2)
-    (vecho "meeting between: ~a & ~a~n" p1 p2) 
-    (%assert! %has-met () ((p1 p2)))
-    (%assert! %has-met () ((p2 p1))))
+    (vecho "meeting between: ~a & ~a~n" p1 p2)
+    (unless (have-they-met? p1 p2)
+      (begin 
+        (%assert! %has-met () ((p1 p2)))
+        (%assert! %has-met () ((p2 p1))))))
 
   ;; various meetings...
   ;;   e.g. (combinations '(p1 p2 p3 p4 p5) 4)
@@ -44,7 +49,7 @@
     (meeting-2 p1 p3)
     (meeting-2 p2 p3))
 
-  (define (meeting-4 p1 p2 p3 p4)
+  (define (meeting-4 p1 p2 p3 p4) ;; '((p1 p2 p3) (p1 p2 p4) (p1 p3 p4) (p2 p3 p4))
     (meeting-3 p1 p2 p3)
     (meeting-3 p1 p2 p4)
     (meeting-3 p1 p3 p4)