def life_range
      if birth
        if death
          birth..death # exact
        else
          birth..(birth + max_le) # estimation based on birth
        end
      elsif death
        (death - max_le)..death # estimation based on death
      end
    end
def life_range
      if birth
        if death
          birth..death # exact
        else
          birth..(birth + max_le) # estimation based on birth
        end
      elsif death
        (death - max_le)..death # estimation based on death
      end
    end
optimistic (longest) estimation of life period. Calculation is based on available dates and max life expectancy
 @return [Range] life period or nil if cannot be computable