basic get&set of Map properties works

src/y.js

@@ -1,11 +1,25 @@
 /* @flow */
 
+const GeneratorFunction = (function*(){}).constructor;
+
 class Y { //eslint-disable-line no-unused-vars
   constructor (opts) {
-    this.connector = new Y[opts.connector.name](opts.connector);
     this.db = new Y[opts.db.name](this, opts.db);
+    this.connector = new Y[opts.connector.name](this, opts.connector);
+    var y = this;
+    this.db.requestTransaction(function*(){
+      yield* this.addOperation({
+        id: ["_", 0],
+        struct: "Map",
+        map: {}
+      });
+      y.root = new Y.Map.Create(["_", 0]);
+    });
   }
   transact (generator) {
+    if (generator.constructor !== GeneratorFunction) {
+      throw new Error("y.transact requires a Generator function! E.g. function*(){/*..*/}");
+    }
     this.db.requestTransaction(generator);
   }
   destroy () {