code: public B addExtensionElement(BpmnModelElementInstance extensionElement) {
      ExtensionElements extensionElements = getCreateSingleChild(ExtensionElements.class);
      extensionElements.addChildElement(extensionElement);
      return myself;
    }
docs: Add an extension element to the element.
  
  @param extensionElement  the extension element to add
  @return the builder object