diff --git a/syndicate/bag.rkt b/syndicate/bag.rkt
index c374e0a..984a651 100644
--- a/syndicate/bag.rkt
+++ b/syndicate/bag.rkt
@@ -5,7 +5,9 @@
          bag-change!
          bag-ref
          bag-clear!
-         in-bag)
+         bag-member?
+         in-bag
+         in-bag/count)
 
 ;; A `(Bagof X)` is a `(MutableHash X Nat)`, where the `Nat` against
 ;; an `X` is its replication count in the bag.
@@ -28,5 +30,7 @@
   (hash-ref b x 0))
 
 (define bag-clear! hash-clear!)
+(define bag-member? hash-has-key?)
 
-(define-syntax-rule (in-bag piece ...) (in-hash piece ...))
+(define-syntax-rule (in-bag piece ...) (in-hash-keys piece ...))
+(define-syntax-rule (in-bag/count piece ...) (in-hash piece ...))
diff --git a/syndicate/dataspace.rkt b/syndicate/dataspace.rkt
index 294465e..0407ab7 100644
--- a/syndicate/dataspace.rkt
+++ b/syndicate/dataspace.rkt
@@ -33,6 +33,7 @@
 
          add-facet!
          stop-facet!
+         add-stop-script! ;; TODO: shouldn't be provided - inline syntax.rkt??
          add-endpoint!
          terminate-facet! ;; TODO: shouldn't be provided - inline syntax.rkt??
          schedule-script! ;; TODO: shouldn't be provided - inline syntax.rkt??
@@ -41,6 +42,9 @@
 
          spawn! ;; TODO: should this be provided?
          enqueue-send! ;; TODO: should this be provided?
+         adhoc-retract! ;; TODO: should this be provided?
+         adhoc-assert! ;; TODO: should this be provided?
+         actor-adhoc-assertions ;; TODO: should this be provided?
          )
 
 (require syndicate/functional-queue)
@@ -76,6 +80,9 @@
 (struct dataspace ([next-id #:mutable] ;; Nat
                    routing-table ;; Skeleton
                    actors ;; (MutableHash ActorID Actor)
+                   ;; v TODO: Caches have to be bags, not sets; once
+                   ;; this change is made, can I avoid keeping a bag
+                   ;; of assertions in the dataspace as a whole?
                    assertions ;; (Bagof Assertion)
                    dataflow ;; DataflowGraph
                    [runnable #:mutable] ;; (Listof Actor)
@@ -88,6 +95,8 @@
                [runnable? #:mutable] ;; Boolean
                pending-scripts ;; (MutableVectorof (Queueof (-> Any)))
                [pending-actions #:mutable] ;; (Queueof Action)
+               ;; TODO: consider using a bag, rather than set, of ad-hoc assertions.
+               [adhoc-assertions #:mutable] ;; (Setof Assertion)
                )
   #:methods gen:custom-write
   [(define (write-proc a p mode)
@@ -212,7 +221,8 @@
                            #f
                            #f
                            (make-vector priority-count (make-queue))
-                           (make-queue)))
+                           (make-queue)
+                           (set)))
   (hash-set! (dataspace-actors ds) the-actor-id the-actor)
   (for [(a initial-assertions)]
     (match (bag-change! (dataspace-assertions ds) a 1)
@@ -312,7 +322,7 @@
     ;; (log-info "performing ~a" action)
     (match action
       [(patch delta)
-       (for [((a count) (in-bag delta))]
+       (for [((a count) (in-bag/count delta))]
          (match (bag-change! (dataspace-assertions ds) a count)
            ['present->absent (remove-assertion! (dataspace-routing-table ds) a)]
            ['absent->present (add-assertion! (dataspace-routing-table ds) a)]
@@ -388,6 +398,8 @@
 ;; Abruptly terminates an entire actor, without running stop-scripts etc.
 (define (terminate-actor! ds the-actor)
   (hash-remove! (dataspace-actors ds) (actor-id the-actor))
+  (push-script! ds the-actor
+                (lambda () (for [(a (actor-adhoc-assertions the-actor))] (retract! the-actor a))))
   (let ((f (actor-root-facet the-actor)))
     (when f
       (let abort-facet! ((f f))
@@ -468,6 +480,16 @@
   (when (not (void? assertion))
     (bag-change! (ensure-patch-action! ac) assertion +1)))
 
+(define (adhoc-retract! ac assertion)
+  (when (not (void? assertion))
+    (set-actor-adhoc-assertions! ac (set-remove (actor-adhoc-assertions ac) assertion))
+    (retract! ac assertion)))
+
+(define (adhoc-assert! ac assertion)
+  (when (not (void? assertion))
+    (set-actor-adhoc-assertions! ac (set-add (actor-adhoc-assertions ac) assertion))
+    (assert! ac assertion)))
+
 (define (dataspace-unsubscribe! ds h)
   (remove-interest! (dataspace-routing-table ds) h))
 
diff --git a/syndicate/pattern.rkt b/syndicate/pattern.rkt
index ddcdf13..cb6fd6e 100644
--- a/syndicate/pattern.rkt
+++ b/syndicate/pattern.rkt
@@ -4,6 +4,7 @@
          (struct-out capture)
 
          (for-syntax analyse-pattern
+                     instantiate-pattern
                      desc->key
                      desc->skeleton-proj
                      desc->skeleton-stx
@@ -76,7 +77,25 @@
        (list 'discard)]
       [_
        (list 'atom stx)]))
-  )
+
+  (define (instantiate-pattern stx)
+    (syntax-case stx ($)
+      [(ctor piece ...)
+       (struct-info? (id-value #'ctor))
+       (quasisyntax/loc stx (ctor #,@(stx-map instantiate-pattern #'(piece ...))))]
+      [(list piece ...)
+       (list-id? #'list)
+       (quasisyntax/loc stx (list #,@(stx-map instantiate-pattern #'(piece ...))))]
+      [id
+       (dollar-id? #'id)
+       (undollar #'id)]
+      [($ id p)
+       #'id]
+      [id
+       (discard-id? #'id)
+       #'id]
+      [other
+       #'other])))
 
 ;;---------------------------------------------------------------------------
 
diff --git a/syndicate/skeleton.rkt b/syndicate/skeleton.rkt
index 0dcf77d..04fe719 100644
--- a/syndicate/skeleton.rkt
+++ b/syndicate/skeleton.rkt
@@ -18,6 +18,8 @@
 (require racket/hash)
 (require racket/list)
 
+(require "bag.rkt")
+
 (module+ test (require rackunit))
 
 ;; A `Skeleton` is a structural guard on an assertion: essentially,
@@ -50,12 +52,12 @@
 ;;                 (MutableSet Assertion)
 ;;                 (MutableHash SkProj SkAcc))
 ;;       SkAcc = (skeleton-accumulator
-;;                 (Set SkKey)
+;;                 (MutableBag SkKey)
 ;;                 (MutableSeteq (... -> Any)))
 ;;
 (struct skeleton-continuation (cache table) #:transparent)
 (struct skeleton-matched-constant (cache table) #:transparent)
-(struct skeleton-accumulator ([cache #:mutable] handlers) #:transparent)
+(struct skeleton-accumulator (cache handlers) #:transparent)
 ;;
 ;; A `SkProj` is a *skeleton projection*, a specification of loci
 ;; within a tree-shaped assertion to collect into a flat list.
@@ -111,12 +113,14 @@
   (define cvt (hash-ref! (skeleton-continuation-table c) cs make-hash))
   (define sc (hash-ref! cvt cv make-matched-constant))
   (define (make-accumulator)
-    (skeleton-accumulator (for/set [(a (skeleton-matched-constant-cache sc))]
-                            (apply-projection a vs))
-                          (mutable-seteq)))
+    (define cache (make-bag))
+    (for [(a (skeleton-matched-constant-cache sc))]
+      (define vars (apply-projection a vs))
+      (bag-change! cache vars 1))
+    (skeleton-accumulator cache (mutable-seteq)))
   (define acc (hash-ref! (skeleton-matched-constant-table sc) vs make-accumulator))
   (set-add! (skeleton-accumulator-handlers acc) h)
-  (for [(vars (skeleton-accumulator-cache acc))] (apply-handler! h vars)))
+  (for [(vars (in-bag (skeleton-accumulator-cache acc)))] (apply-handler! h vars)))
 
 (define (skcont-remove! c i apply-handler!)
   (match-define (skeleton-interest _desc cs cv vs h) i)
@@ -126,7 +130,7 @@
     (when sc
       (define acc (hash-ref (skeleton-matched-constant-table sc) vs #f))
       (when acc
-        (for [(vars (skeleton-accumulator-cache acc))] (apply-handler! h vars))
+        (for [(vars (in-bag (skeleton-accumulator-cache acc)))] (apply-handler! h vars))
         (set-remove! (skeleton-accumulator-handlers acc) h)
         (when (set-empty? (skeleton-accumulator-handlers acc))
           (hash-remove! (skeleton-matched-constant-table sc) vs)))
@@ -229,9 +233,13 @@
 (define (add-term-to-skconst! skconst term)
   (set-add! (skeleton-matched-constant-cache skconst) term))
 (define (add-term-to-skacc! skacc vars _term)
-  (set-skeleton-accumulator-cache! skacc (set-add (skeleton-accumulator-cache skacc) vars))
-  (for [(handler (skeleton-accumulator-handlers skacc))]
-    (apply handler '+ vars)))
+  (match (bag-change! (skeleton-accumulator-cache skacc) vars 1)
+    ['absent->present
+     (for [(handler (skeleton-accumulator-handlers skacc))]
+       (apply handler '+ vars))]
+    ;; 'present->absent and 'absent->absent absurd
+    ['present->present
+     (void)]))
 
 (define (add-assertion! sk term)
   (skeleton-modify! sk
@@ -245,9 +253,16 @@
 (define (remove-term-from-skconst! skconst term)
   (set-remove! (skeleton-matched-constant-cache skconst) term))
 (define (remove-term-from-skacc! skacc vars _term)
-  (set-skeleton-accumulator-cache! skacc (set-remove (skeleton-accumulator-cache skacc) vars))
-  (for [(handler (skeleton-accumulator-handlers skacc))]
-    (apply handler '- vars)))
+  (define cache (skeleton-accumulator-cache skacc))
+  (if (bag-member? cache vars)
+      (match (bag-change! cache vars -1)
+        ['present->absent
+         (for [(handler (skeleton-accumulator-handlers skacc))]
+           (apply handler '- vars))]
+        ;; 'absent->absent and 'absent->present absurd
+        ['present->present
+         (void)])
+      (log-warning "Removing assertion not previously added: ~v" _term)))
 
 (define (remove-assertion! sk term)
   (skeleton-modify! sk
diff --git a/syndicate/syntax-classes.rkt b/syndicate/syntax-classes.rkt
index e67de9b..acddb82 100644
--- a/syndicate/syntax-classes.rkt
+++ b/syndicate/syntax-classes.rkt
@@ -13,7 +13,7 @@
 (begin-for-syntax
   (define-splicing-syntax-class assertions
     (pattern (~seq #:assertions [exprs ...]))
-    (pattern (~seq) #:attr (exprs 1) #'()))
+    (pattern (~seq) #:attr (exprs 1) '()))
 
   (define-splicing-syntax-class name
     (pattern (~seq #:name N))
diff --git a/syndicate/syntax.rkt b/syndicate/syntax.rkt
index d4a57e6..7d24a8f 100644
--- a/syndicate/syntax.rkt
+++ b/syndicate/syntax.rkt
@@ -17,7 +17,7 @@
          on-start
          on-stop
          on
-         ;; during
+         during
          ;; during/spawn
          begin/dataflow
          define/dataflow
@@ -47,6 +47,9 @@
 
          send!
          flush!
+         assert!
+         retract!
+         current-adhoc-assertions
 
          ;;
 
@@ -357,17 +360,14 @@
                                                 (lambda ()
                                                   #,script-stx))))))))))
 
-;; (define-syntax (during stx)
-;;   (syntax-parse stx
-;;     [(_ P O ...)
-;;      (define E-stx (syntax/loc #'P (asserted P)))
-;;      (define-values (_proj _pat _bindings instantiated)
-;;        (analyze-pattern E-stx #'P))
-;;      (quasisyntax/loc stx
-;;        (on #,E-stx
-;;            (let ((p #,instantiated))
-;;              (react (stop-when (retracted p))
-;;                     O ...))))]))
+(define-syntax (during stx)
+  (syntax-parse stx
+    [(_ P O ...)
+     (define Q-stx (instantiate-pattern #'P))
+     (quasisyntax/loc stx
+       (on (asserted P)
+           (react (stop-when (retracted #,Q-stx))
+                  O ...)))]))
 
 ;; (define-syntax (during/spawn stx)
 ;;   (syntax-parse stx
@@ -576,6 +576,17 @@
   (until (message ack)
     (on-start (send! ack))))
 
+(define (assert! a)
+  (ensure-in-script! 'assert!)
+  (adhoc-assert! (current-actor) a))
+
+(define (retract! a)
+  (ensure-in-script! 'retract!)
+  (adhoc-retract! (current-actor) a))
+
+(define (current-adhoc-assertions)
+  (actor-adhoc-assertions (current-actor)))
+
 ;;---------------------------------------------------------------------------
 
 (module+ test
@@ -589,6 +600,71 @@
        (schedule-script!
         (current-dataspace)
         (current-actor)
+
+        #;(lambda ()
+          (struct foo (x y) #:prefab)
+
+          (spawn (field [x 123])
+                 (assert (foo (x) 999))
+                 (during (foo (x) $v)
+                   (log-info "x=~a v=~a" (x) v)
+                   (when (= (x) 123) (x 124))
+                   (on-stop
+                    (log-info "finally for x=~a v=~a" (x) v))))
+          )
+
+        (lambda ()
+          ;; .../racket/syndicate/examples/actor/example-partial-retraction.rkt
+          ;;
+          (struct ready (what) #:prefab)
+          (struct entry (key val) #:prefab)
+
+          (spawn (assert (ready 'listener))
+                 (on (asserted (entry $key _))
+                     (log-info "key ~v asserted" key)
+                     (until (retracted (entry key _))
+                       (on (asserted (entry key $value))
+                           (log-info "add binding: ~v -> ~v" key value))
+                       (on (retracted (entry key $value))
+                           (log-info "del binding: ~v -> ~v" key value)))
+                     (log-info "key ~v retracted" key)))
+
+          (spawn (assert (ready 'other-listener))
+                 (during (entry $key _)
+                   (log-info "(other-listener) key ~v asserted" key)
+                   (on-stop (log-info "(other-listener) key ~v retracted" key))
+                   (during (entry key $value)
+                     (log-info "(other-listener) ~v ---> ~v" key value)
+                     (on-stop (log-info "(other-listener) ~v -/-> ~v" key value)))))
+
+          (define (pause)
+            (log-info "pause")
+            (define token (gensym 'pause)) ;; FIXME:: If we use the same token every time, need epochs!
+            (until (asserted (ready token))
+              (assert (ready token))))
+
+          (spawn* (until (asserted (ready 'listener)))
+                  (until (asserted (ready 'other-listener)))
+                  (assert! (entry 'a 1))
+                  (assert! (entry 'a 2))
+                  (assert! (entry 'b 3))
+                  (assert! (entry 'c 33))
+                  (assert! (entry 'a 4))
+                  (assert! (entry 'a 5))
+                  (pause)
+                  (retract! (entry 'a 2))
+                  (retract! (entry 'c 33))
+                  (assert! (entry 'a 9))
+                  (pause)
+                  (for [(a (current-adhoc-assertions))]
+                    (local-require racket/match)
+                    (match a
+                      [(entry 'a _) (retract! a)]
+                      [_ (void)]))
+                  ;; ^ (retract! (entry 'a ?))
+                  (pause))
+          )
+
         #;(lambda ()
           (spawn (on (message $v)
                      (if (= v 10000000)
@@ -597,7 +673,7 @@
                  (on-start (send! 0)))
           )
 
-        (lambda ()
+        #;(lambda ()
 
           (message-struct stage (n))