code: public <T extends AbstractHtml> T findOneTagAssignableToTag(
              final Class<T> tagClass) throws NullValueException {
          return findOneTagAssignableToTag(false, tagClass);
      }
docs: Finds and returns the first matching (including from nested tags) tag
  (which is assignable to the given tag class). <br>
  <br>
  
  <pre>
  <code>
  Html html = new Html(null) {{
  new Head(this) {{
  new TitleTag(this){{
  new NoTag(this, "some title");
  }};
  }};
  new Body(this, new Id("one")) {{
  new Div(this);
  }};
  }};
  
  TitleTag titleTag = TagRepository.findOneTagAssignableToTagClass(TitleTag.class, html);
  
  System.out.println(titleTag.getTagName());
  System.out.println(titleTag.toHtmlString());
  
  //prints
  
  title
  &lt;title&gt;some title&lt;/title&gt;
  
  </code>
  </pre>
  
  @param tagClass
  the class of the tag.
  @return the first matching tag which is assignable to the given tag
  class.
  @throws NullValueException
  if the {@code tagClass} is null
  @since 2.1.11
  @author WFF