(ml5717) Union struct first merge sync fix

src/Struct.js

@@ -165,7 +165,7 @@ module.exports = function (Y/* :any */) {
           o = (o.right == null) ? null : yield* this.getOperation(o.right)
         } else { // left == null
           parent = yield* this.getOperation(op.parent)
-          let startId = op.parentSub ? parent.map[op.parentSub] : parent.start
+          let startId = op.parentSub ? parent.map[op.parentSub] : (parent.struct == "Union" ? parent.union : parent.start)
           start = startId == null ? null : yield* this.getOperation(startId)
           o = start
         }
@@ -223,7 +223,7 @@ module.exports = function (Y/* :any */) {
           yield* this.setOperation(left)
         } else {
           // set op.right from parent, if necessary
-          op.right = op.parentSub ? parent.map[op.parentSub] || null : parent.start
+          op.right = op.parentSub ? parent.map[op.parentSub] || null : (parent.struct == "Union" ? parent.union : parent.start)
         }
         // reconnect right
         if (op.right != null) {
@@ -256,6 +256,11 @@ module.exports = function (Y/* :any */) {
           if (op.left != null) {
             yield* this.deleteOperation(op.id, 1, true)
           }
+        } else if (parent.struct == "Union") {
+          if (left == null) {
+            parent.union = op.id
+            yield* this.setOperation(parent)
+          }
         } else {
           if (right == null || left == null) {
             if (right == null) {