code: protected function renderButtons(array $aButtons, $bJustified = false)
      {
          $sMarkup = '';
          foreach ($aButtons as $oButton) {
              if (is_array($oButton) ||
                  ($oButton instanceof Traversable &&
                  !($oButton instanceof ElementInterface))
              ) {
                  $oFactory = new Factory();
                  $oButton = $oFactory->create($oButton);
              } elseif (!($oButton instanceof ElementInterface)) {
                  throw new LogicException(sprintf(
                      'Button expects an instanceof Zend\Form\ElementInterface or an array / Traversable, "%s" given', is_object($oButton) ? get_class($oButton) : gettype($oButton)
                  ));
              }
  
              $sButtonMarkup = $this->getFormElementHelper()->__invoke($oButton);
  
              $sMarkup .= $bJustified ? sprintf(static::$buttonGroupJustifiedFormat, $sButtonMarkup) : $sButtonMarkup;
          }
          return $sMarkup;
      }
docs: Render buttons markup
  @param array $aButtons
  @return string