protected function nspKey(string $primary_key): string
    {
        if (!$this->enabled) {
            return ''; // Not possible.
            //
        } elseif (!isset($primary_key[0])) {
            return ''; // Not possible.
        }
        $namespace_prefix       = 'x___'.$this->namespace.'\\';
        $namespaced_primary_key = $namespace_prefix.$primary_key;

        if (isset($namespaced_primary_key[251])) {
            throw $this->c::issue(sprintf('Primary key too long; %1$s bytes max.', 250 - strlen($namespace_prefix)));
        }
        return $namespaced_primary_key;
    }