function append(element) {
    element && elements.push(element);
  }
@ngdoc directive
@name ng.directive:ngApp

@element ANY
@param {angular.Module} ngApp an optional application
  {@link angular.module module} name to load.

@description

Use this directive to auto-bootstrap an application. Only
one ngApp directive can be used per HTML document. The directive
designates the root of the application and is typically placed
at the root of the page.

The first ngApp found in the document will be auto-bootstrapped. To use multiple applications in
an HTML document you must manually bootstrap them using {@link angular.bootstrap}.
Applications cannot be nested.

In the example below if the `ngApp` directive were not placed
on the `html` element then the document would not be compiled
and the `{{ 1+2 }}` would not be resolved to `3`.

`ngApp` is the easiest way to bootstrap an application.

 <doc:example>
   <doc:source>
    I can add: 1 + 2 =  {{ 1+2 }}
   </doc:source>
 </doc:example>