public function withPopulation(string $population = null): self
    {
        $new = clone $this;
        $new->population = null === $population ? null : (int) $population;

        return $new;
    }
public function withPopulation(string $population = null): self
    {
        $new = clone $this;
        $new->population = null === $population ? null : (int) $population;

        return $new;
    }
@param int|null $population

@return CountryInfo