Allow multiple "root" facets, by creating a dummy root

syndicate/dataspace.rkt

@@ -265,11 +265,17 @@
       ['absent->present (add-assertion! (dataspace-routing-table ds) a)]
       ;; 'absent->absent and 'present->absent absurd
       ['present->present (void)])) ;; i.e. no visible change
+  ;; Root facet is a dummy "system" facet that exists to hold one-or-more "user" "root" facets.
   (add-facet! #f
               the-actor
               #f
               (lambda ()
-                (boot-proc)
+                ;; The "true root", user-visible facet.
+                (add-facet! #f
+                            the-actor
+                            (current-facet)
+                            (lambda ()
+                              (boot-proc)))
                 (for [(a filtered-initial-assertions)]
                   (adhoc-retract! the-actor a)))))
 
@@ -410,9 +416,12 @@
       (set-facet-children! parent (set-add (facet-children parent) f))
       (begin
         (when (actor-root-facet actor)
-          ;; TODO: consider whether this is legit. For example, (stop-facet the-root-facet
-          ;; (react ...) (react ...)) should probably be admissible, so perhaps more than one
-          ;; root facet should be allowed?
+          ;; This should never happen. We deliberately create an
+          ;; otherwise-dummy root facet for each actor specifically to
+          ;; hold user facets, and there should be no way for the user
+          ;; to stop that root facet explicitly, which means user code
+          ;; can't start any replacements for it at all, let alone
+          ;; more than one!
           (error 'add-facet! "INTERNAL ERROR: Attempt to add second root facet"))
         (set-actor-root-facet! actor f)))
   (with-current-facet [f]