diff --git a/gallery/example1.pdf b/gallery/example1.pdf
index a1d2b15..1042c9a 100644
Binary files a/gallery/example1.pdf and b/gallery/example1.pdf differ
diff --git a/gallery/example2.pdf b/gallery/example2.pdf
index 174b24b..69a5943 100644
Binary files a/gallery/example2.pdf and b/gallery/example2.pdf differ
diff --git a/gallery/example2.typ b/gallery/example2.typ
index 2b50105..ca2903e 100644
--- a/gallery/example2.typ
+++ b/gallery/example2.typ
@@ -39,3 +39,13 @@
   _seq("charlie", "alice", comment: "ok", dashed: true, disable-src: true)
 })
 
+#chronos.diagram({
+  import chronos: *
+  _seq("?", "Alice", comment: [?->\ *short* to actor1])
+  _seq("[", "Alice", comment: [\[->\ *from start* to actor1])
+  _seq("[", "Bob", comment: [\[->\ *from start* to actor2])
+  _seq("?", "Bob", comment: [?->\ *short* to actor2])
+  _seq("Alice", "]", comment: [->\]\ from actor1 *to end*])
+  _seq("Alice", "?", comment: [->?\ *short* from actor1])
+  _seq("Alice", "Bob", comment: [->\ from actor1 to actor2])
+})
\ No newline at end of file
diff --git a/src/diagram.typ b/src/diagram.typ
index 916b5c7..26866e1 100644
--- a/src/diagram.typ
+++ b/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 + "?"
+      }
     }
   }
 
diff --git a/src/participant.typ b/src/participant.typ
index 4ff6761..f661d4c 100644
--- a/src/participant.typ
+++ b/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
diff --git a/src/renderer.typ b/src/renderer.typ
index 531fc37..d8b1504 100644
--- a/src/renderer.typ
+++ b/src/renderer.typ
@@ -2,10 +2,12 @@
 #import "utils.typ": get-participants-i, get-style
 #import "group.typ"
 #import "participant.typ"
+#import participant: PAR-SPECIALS
 #import "sequence.typ"
 #import "separator.typ"
 #import "consts.typ": *
 
+#let DEBUG-INVISIBLE = false
 
 #let get-columns-width(participants, elements) = {
   participants = participants.map(p => {
@@ -56,8 +58,10 @@
   for i in range(participants.len() - 1) {
     let p1 = participants.at(i)
     let p2 = participants.at(i + 1)
-    let w1 = measure(box(p1.display-name)).width + PAR-PAD.last() * 2
-    let w2 = measure(box(p2.display-name)).width + PAR-PAD.last() * 2
+    let w1 = if p1.invisible {0pt} else {measure(box(p1.display-name)).width}
+    let w2 = if p2.invisible {0pt} else {measure(box(p2.display-name)).width}
+    w1 += PAR-PAD.last() * 2
+    w2 += PAR-PAD.last() * 2
     widths.push(w1 / 2pt + w2 / 2pt + PAR-SPACE)
   }
 
@@ -79,7 +83,7 @@
     let m = measure(cell.cell)
     widths.at(cell.i2 - 1) = calc.max(
       widths.at(cell.i2 - 1),
-      m.width / 1pt - widths.slice(0, cell.i2 - 1).sum()
+      m.width / 1pt - widths.slice(cell.i1, cell.i2 - 1).sum()
     )
   }
 
@@ -115,7 +119,7 @@
   
   // Draw participants (start)
   for p in participants {
-    if p.from-start {
+    if p.from-start and not p.invisible {
       shapes += draw-par(p)
     }
   }
@@ -174,8 +178,26 @@
 
   // Draw vertical lines + lifelines + end participants
   shapes += draw.on-layer(-1, {
-    for (i, p) in participants.enumerate() {
-      let x = x-pos.at(i)
+    if DEBUG-INVISIBLE {
+      for p in participants.filter(p => p.invisible) {
+        let color = if p.name.starts-with("?") {green} else if p.name.ends-with("?") {red} else {blue}
+        let x = x-pos.at(p.i)
+        draw.line(
+          (x, 0),
+          (x, y),
+          stroke: (paint: color, dash: "dotted")
+        )
+        draw.content(
+          (x, 0),
+          p.display-name,
+          anchor: "west",
+          angle: 90deg
+        )
+      }
+    }
+    
+    for p in participants.filter(p => not p.invisible) {
+      let x = x-pos.at(p.i)
 
       // Draw vertical line
       let last-y = 0
@@ -185,7 +207,7 @@
       let lines = ()
 
       // Compute lifeline rectangles + destruction positions
-      for line in lifelines.at(i).lines {
+      for line in lifelines.at(p.i).lines {
         let event = line.first()
         if event == "create" {
           last-y = line.at(1)