public function withAreaInSqKm(string $areaInSqKm = null): self
    {
        $new = clone $this;
        $new->areaInSqKm = null === $areaInSqKm ? null : (float) $areaInSqKm;

        return $new;
    }
public function withAreaInSqKm(string $areaInSqKm = null): self
    {
        $new = clone $this;
        $new->areaInSqKm = null === $areaInSqKm ? null : (float) $areaInSqKm;

        return $new;
    }
@param string|null $areaInSqKm

@return CountryInfo