fixed new participants not detected in syncs

fixes #20

src/core/setup.typ

@@ -1,4 +1,5 @@
 #import "draw/group.typ": render-end as grp-render-end
+#import "draw/sync.typ": render-end as sync-render-end
 #import "utils.typ": get-group-span, is-elmt
 #import "/src/participant.typ": _exists as par-exists, _par
 #import "/src/sequence.typ": _seq
@@ -18,6 +19,24 @@
   )
 }
 
+#let flatten-sync(elmts, i) = {
+  let sync = elmts.at(i)
+  elmts.at(i) = sync
+  let start = sync
+  start.remove("elmts")
+  return (
+    elmts.slice(0, i) +
+    (start,) +
+    sync.elmts +
+    ((
+      type: "sync-end",
+      draw: sync-render-end,
+      elmts: sync.elmts
+    ),) +
+    elmts.slice(i + 1)
+  )
+}
+
 #let update-group-children(elmts, i) = {
   let elmts = elmts
   let group-end = elmts.at(i)
@@ -58,6 +77,9 @@
 
     if elmt.type == "grp" {
       elmts = flatten-group(elmts, i)
+    
+    } else if elmt.type == "sync" {
+      elmts = flatten-sync(elmts, i)
 
     } else if elmt.type == "seq" {
       if elmt.enable-dst {