Q:

How to use existing widget form template blocks

I am creating custom Twig form template blocks to do some special rendering.
I have been unable, so far, to use some of the built-in form blocks. Specifically, and of the {{ *_widget() }} blocks.
Example:
Declare a new Twig function.  
/* src/SiteBundle/Library/TwigExtension.php - Bundle-specific Twig extension */
...
public function getFunctions
{
  $ret = [
    new \Twig_SimpleFunction( 'wwui_myBlock', null, ['node_class'=>'Symfony\Bridge\Twig\Node\RenderBlockNode, 'is_safe'=>['html']] )
  ];
  return $ret;
}

The extension is registered in the service.yml file and is obviously recognized since there are no errors about the wwui_myBlock function being undefined.
Define the Twig block.  
{# src/SiteBundle/Resources/views/Form/fields.html.twig - Custom form blocks #}
{% block wwui_myBlock %}
  <div class="formRow form-group-xs clearfix">
    <div class="col-xs-1"></div>
    {%- set label_attr = label_attr|merge({class: (label_attr.class|default('') ~ ' col-xs-2' )|trim}) -%}
    {{ form_label( form ) }}
    <div class="col-xs-9">
      {{ choice_widget( form ) }}
    </div>
  </div>
{% endblock wwui_myBlock %}

In the page template.
{# Page-specific template #}
...
  {{ form_start( form ) }}
    ...
    {{ wwui_myBlock( form.options ) }} {# options is a choice with radio buttons. #}
    ...
  {{ form_end( form ) }}

When this page is rendered an exception is thrown:
The function "choice_widget" does not exist in SiteBundle:Form:fields.html.twig at line 62
My fields.html.twig file "uses" bootstrap_3_horizontal_layout.html.twig. That file "uses" bootstrap_3_layout.html.twig which, in turn, "uses" Twig's default form_div_layout.html.twig. 
Why can't my custom block see choice_widget which is defined in form_div_layout.html.twig?

A:

Turns out that the proper way to access such blocks is to use the block() function. so, instead of 
{{ choice_widget( form ) }}

it's 
{{ block( 'choice_widget', form ) }}

