docs: function(arg) {
      readOnly(this, ID, COUNT++);
      readOnly(this, "_listeners", new Listeners());
      readOnly(this, "_links", []);
      // An object  with the  attribute `isContentChangeAware  === true`
      // can fire a property value changed when its contant has change.
      readOnly(this, "isContentChangeAware", true);
      if (Array.isArray(arg)) {
          readOnly(this, "_array", arg);
      } else if (isList(arg)) {
          readOnly(this, "_array", arg._array);
          this.link(arg);
      } else {
          readOnly(this, "_array", [arg]);
      }
  }