added short/start/end arrows

src/participant.typ

@@ -1,16 +1,23 @@
 #import "@preview/cetz:0.2.2": draw
 #import "consts.typ": *
 
-#let _par(name, display-name: auto, from-start: true) = {
+#let PAR-SPECIALS = "?[]"
+
+#let _par(name, display-name: auto, from-start: true, invisible: false) = {
   return ((
     type: "par",
     name: name,
     display-name: if display-name == auto {name} else {display-name},
-    from-start: from-start
+    from-start: from-start,
+    invisible: invisible
   ),)
 }
 
 #let _exists(participants, name) = {
+  if name == "?" or name == "[" or name == "]" {
+    return true
+  }
+
   for p in participants {
     if name == p.name {
       return true