public function mustBeInstanceOf(string $instanceOf) : Slot
    {
        $this->instanceOf = class_exists($instanceOf) || interface_exists($instanceOf) ? $instanceOf : self::INSTANCE_OF_ANY;
        return $this;
    }
public function mustBeInstanceOf(string $instanceOf) : Slot
    {
        $this->instanceOf = class_exists($instanceOf) || interface_exists($instanceOf) ? $instanceOf : self::INSTANCE_OF_ANY;
        return $this;
    }
public function mustBeInstanceOf ( string $ instanceOf ) : Slot { $ this -> instanceOf = class_exists ( $ instanceOf ) || interface_exists ( $ instanceOf ) ? $ instanceOf : self :: INSTANCE_OF_ANY ; return $ this ; }
Set instance-of restriction

@param string $instanceOf

@return Slot
Set instance - of restriction