grouped some elmt defs in misc.typ

src/participant.typ

@@ -1,6 +1,6 @@
 #import "core/draw/participant.typ"
 
-#let PAR-SPECIALS = "?[]"
+#let PAR-SPECIALS = ("?", "[", "]")
 #let SHAPES = (
   "participant",
   "actor",
@@ -50,7 +50,7 @@
 }
 
 #let _exists(participants, name) = {
-  if name == "?" or name == "[" or name == "]" {
+  if name in PAR-SPECIALS {
     return true
   }