added short/start/end arrows

src/diagram.typ

@@ -1,6 +1,6 @@
 #import "utils.typ": get-group-span
 #import "renderer.typ": render
-#import "participant.typ" as participant: _par
+#import "participant.typ" as participant: _par, PAR-SPECIALS
 
 #let _gap(size: 20) = {
   return ((
@@ -45,6 +45,22 @@
     }
   }
 
+  let pars = participants
+  participants = ()
+  participants.push(_par("[", invisible: true).first())
+  for (i, p) in pars.enumerate() {
+    let before = _par("?" + p.name, invisible: true).first()
+    let after = _par(p.name + "?", invisible: true).first()
+    if i == 0 {
+      participants.push(before)
+    } else {
+      participants.insert(-1, before)
+    }
+    participants.push(p)
+    participants.push(after)
+  }
+  participants.push(_par("]", invisible: true).first())
+
   // Add index to participant
   for (i, p) in participants.enumerate() {
     p.insert("i", i)
@@ -57,6 +73,12 @@
       let (min-i, max-i) = get-group-span(participants, elmt)
       elmts.at(i).insert("min-i", min-i)
       elmts.at(i).insert("max-i", max-i)
+    } else if elmt.type == "seq" {
+      if elmt.p1 == "?" {
+        elmts.at(i).p1 = "?" + elmt.p2
+      } else if elmt.p2 == "?" {
+        elmts.at(i).p2 = elmt.p1 + "?"
+      }
     }
   }