kernel/lib/Form/Container.php
private function _validate($oElement) : bool
    {
        if (is_array($oElement->getConstraint())) {

            foreach ($oElement->getConstraint() as $oConstraint) {

                if (!$oConstraint->validate($_POST[$oElement->getName()])) {

                    return false;
                }
            }
        }

        return true;
    }
private function _validate($oElement) : bool
    {
        if (is_array($oElement->getConstraint())) {

            foreach ($oElement->getConstraint() as $oConstraint) {

                if (!$oConstraint->validate($_POST[$oElement->getName()])) {

                    return false;
                }
            }
        }

        return true;
    }
if the element is valide or not (with the constraint)

@access private
@param  object $oElement element of formular
@return boolean