private final function addForeign(KeyInterface $key, string $name): string
    {
        return "ADD CONSTRAINT" . " `fk_" . $name . "`"
             . " " . $this->constant($key->getType())
             . " (`" . implode("`, `", ($key->getSubject())) . "`) "
             . " REFERENCES `" . $key->getForeigner() . "`"
             . " (`" . implode("`, `", ($key->getForeignerSubject())) . "`),\n";
    }