Hack a Parcel.WHICH solution

Apparently, a Parcel created by the .Parcel method, consists of a ListIter
as its only element.  Upon inspection, this is flattened, causing a problem
with the .WHICH representation, as the immutable Parcel internally mutes.
This fix will just expose the ListIter if the Parcel was created with
.Parcel, and thereby create a WHICH that indicates the inherit mutability
of a Parcel that is created that way.  Suggestions for a better solution
are welcome.

src/core/Parcel.pm

@@ -11,8 +11,9 @@ my class Parcel does Positional { # declared in BOOTSTRAP
     multi method ACCEPTS(Parcel:D: $topic) { self.list.ACCEPTS($topic) }
 
     multi method WHICH(Parcel:D:) {
-        $!WHICH //=
-          self.^name ~ '|' ~ @(self).map( {'(' ~ .VAR.WHICH ~ ')'} ).join;
+        $!WHICH //= self[0] ~~ ListIter  # XXX hacky fix for .Parcel parcels
+          ?? self.^name ~ '|(' ~ self[0].VAR.WHICH ~ ')'
+          !! self.^name ~ '|' ~ @(self).map( {'(' ~ .VAR.WHICH ~ ')'} ).join;
     }
     method Parcel()  { self }
     method Capture() {