Fix bug exposed by previous commit

syndicate/HOWITWORKS.md

@@ -295,7 +295,7 @@ cases where handlers are dynamically installed.
                       classof (project v h) = class }
               edges[selector][class] := ((innercache, {}), {})
           let node' = edges[selector][class]
-          let h' = update-path h n_pop 0
+          let h' = update-path h 0 0
           walk-edge h' node' 0 0 [s_0, ..., s_i]
 
         update-path :: H -> 𝐍 -> 𝐍 -> H

syndicate/skeleton.rkt

@@ -212,7 +212,7 @@
                             (hash-set! filtered a #t))))
          (make-empty-skeleton/cache filtered))
        (define next (hash-ref! table class make-skeleton-node-with-cache))
-       (walk-edge! (update-path path pop-count 0) next 0 0 pieces)]
+       (walk-edge! (update-path path 0 0) next 0 0 pieces)]
       [_
        (values pop-count sk)]))
   (define (walk-edge! path sk pop-count index pieces)

syndicate/test/core/complex-pattern.rkt

@@ -0,0 +1,26 @@
+#lang imperative-syndicate/test-implementation
+
+(test-case
+    [(message-struct b (v))
+     (message-struct c (v))
+     (spawn #:name 'respondent
+            (on (asserted ($ val (list _ (vector _) (b (c _)) (vector _) _)))
+                ;; ^ exercises a corner-case in `extend-skeleton!`
+                ;; that exposed why `(update-path path pop-count 0)`
+                ;; was wrong, and `(update-path path 0 0)` was right.
+                (send! 'ok)))]
+  no-crashes)
+
+;;
+;; Trie steps:
+;;
+;;   ·
+;;   - () must be list/5
+;;   pop 0, get 1
+;;   - (1) must be vector/1
+;;   pop 1, get 2
+;;   - (2) must be b/1
+;;   pop 0, get 0
+;;   - (2 0) must be c/1
+;;   pop 2, get 3
+;;   - (3) must be vector/1