diff --git a/syndicate/syntax.rkt b/syndicate/syntax.rkt
index 9d74e4f..767722a 100644
--- a/syndicate/syntax.rkt
+++ b/syndicate/syntax.rkt
@@ -23,10 +23,12 @@
 
          begin/dataflow
          define/dataflow
+         stop-when-true
 
          this-target
          at
          assert
+         stop-when
          (rename-out [event:when when])
          during
          during*)
@@ -116,6 +118,11 @@
   (begin (define-field id #f)
          (begin/dataflow (id expr))))
 
+(define-syntax-rule (stop-when-true test expr ...)
+  (begin/dataflow
+    (when test
+      (stop-current-facet expr ...))))
+
 ;;---------------------------------------------------------------------------
 
 (define-for-syntax orig-insp
@@ -146,11 +153,16 @@
     [(_ expr)
      (turn-assert/dataflow! this-turn this-target (action () (:template expr)))]))
 
+(define-event-expander stop-when
+  (syntax-rules ()
+    [(_ event expr ...)
+     (event:when event (stop-current-facet expr ...))]))
+
 (require "schemas/gen/dataspace.rkt")
 
 (define-event-expander event:when
   (lambda (stx)
-    (syntax-case stx (message asserted)
+    (syntax-case stx (message asserted retracted)
       [(_ (message pat) expr ...)
        #`(assert (Observe (:pattern pat)
                           (ref (entity #:message
@@ -162,7 +174,19 @@
                           (ref (entity #:assert
                                        (action (bindings _handle)
                                          (match-define (list #,@(analyse-pattern-bindings #'pat)) bindings)
-                                         expr ...)))))]))
+                                         expr ...)))))]
+      [(_ (retracted pat) expr ...)
+       #`(assert (Observe (:pattern pat)
+                          (let ((assertion-map (make-hash)))
+                            (ref (entity #:assert
+                                         (action (bindings handle)
+                                           (hash-set! assertion-map handle bindings))
+                                         #:retract
+                                         (action (handle)
+                                           (match-define (list #,@(analyse-pattern-bindings #'pat))
+                                             (hash-ref assertion-map handle))
+                                           (hash-remove! assertion-map handle)
+                                           expr ...))))))]))
   (syntax-rules ()
     [(_ test expr ...)
      (when test expr ...)]))