added _ret for return sequences

src/diagram.typ

@@ -1,6 +1,7 @@
 #import "utils.typ": get-group-span, fit-canvas
 #import "renderer.typ": render
 #import "participant.typ" as participant: _par, PAR-SPECIALS
+#import "sequence.typ": _seq
 
 #let _gap(size: 20) = {
   return ((
@@ -38,7 +39,9 @@
   let elmts = elements
   let i = 0
 
-  // Flatten groups
+  let activation-history = ()
+
+  // Flatten groups + convert returns
   while i < elmts.len() {
     let elmt = elmts.at(i)
     if elmt.type == "grp" {
@@ -62,6 +65,30 @@
         ),) +
         elmts.slice(i+1)
       )
+    } else if elmt.type == "seq" {
+      if elmt.enable-dst {
+        activation-history.push(elmt)
+      }
+    } else if elmt.type == "evt" {
+      if elmt.event == "enable" {
+        for elmt2 in elmts.slice(0, i).rev() {
+          if elmt2.type == "seq" {
+            activation-history.push(elmt2)
+            break
+          }
+        }
+      }
+    } else if elmt.type == "ret" {
+      if activation-history.len() == 0 {
+        panic("Cannot return if no lifeline is activated")
+      }
+      let seq = activation-history.pop()
+      elmts.at(i) = _seq(
+        seq.p2, seq.p1,
+        comment: elmt.comment,
+        disable-src: true,
+        dashed: true
+      ).first()
     }
     i += 1
   }