added synched sequences

src/sync.typ

@@ -0,0 +1,27 @@
+#import "sequence.typ"
+
+#let _sync(elmts) = {
+  return ((
+    type: "sync",
+    elmts: elmts
+  ),)
+}
+
+#let render(pars-i, x-pos, participants, elmt, y, lifelines) = {
+  let draw-seq = sequence.render.with(pars-i, x-pos, participants)
+
+  let shapes = ()
+
+  let end-y = y
+
+  for e in elmt.elmts {
+    let yi
+    let shps
+    (yi, lifelines, shps) = draw-seq(e, y, lifelines)
+    shapes += shps
+    end-y = calc.min(end-y, yi)
+  }
+
+  let r = (end-y, lifelines, shapes)
+  return r
+}