added support for auto color with _note and _par

src/sequence.typ

@@ -4,6 +4,9 @@
 #import "note.typ"
 
 #let get-arrow-marks(sym, color) = {
+  if sym == none {
+    return none
+  }
   if type(sym) == array {
     return sym.map(s => get-arrow-marks(s, color))
   }