added _ret for return sequences

src/sequence.typ

@@ -88,6 +88,13 @@
   ),)
 }
 
+#let _ret(comment: none) = {
+  return ((
+    type: "ret",
+    comment: comment
+  ),)
+}
+
 #let render(pars-i, x-pos, participants, elmt, y, lifelines) = {
   let shapes = ()