public function getReduceRule(): ?Rule
    {
        foreach ($this->items as $item) {
            if (!$item->hasFurther()) {
                return $item->getAsRule();
            }
        }
        return null;
    }
public function getReduceRule(): ?Rule
    {
        foreach ($this->items as $item) {
            if (!$item->hasFurther()) {
                return $item->getAsRule();
            }
        }
        return null;
    }
public function getReduceRule ( ) : ? Rule { foreach ( $ this -> items as $ item ) { if ( ! $ item -> hasFurther ( ) ) { return $ item -> getAsRule ( ) ; } } return null ; }
Get reduction rule of this set if any
@return Rule|null
Get reduction rule of this set if any