function importValue($value)
 {
     if (!in_array($value, $this->getAvailableValues())) {
         $value = null;
         $this->setError(FormControlError::invalid());
     }
     if (!$value && !$this->isOptional()) {
         $this->setError(FormControlError::missing());
     }
     return parent::importValue($value);
 }
 function __construct($name, $label)
 {
     Assert::isTrue(preg_match(FormControlSet::NAME_PATTERN, $name), 'name of a set should contain alphanumeric symbols, glyphs and underscored only');
     parent::__construct($name, $label);
 }