def +(other)
      if Duration === other
        parts = @parts.dup
        other.parts.each do |(key, value)|
          parts[key] += value
        end
        Duration.new(value + other.value, parts)
      else
        seconds = @parts[:seconds] + other
        Duration.new(value + other, @parts.merge(seconds: seconds))
      end
    end
def +(other)
      if Duration === other
        parts = @parts.dup
        other.parts.each do |(key, value)|
          parts[key] += value
        end
        Duration.new(value + other.value, parts)
      else
        seconds = @parts[:seconds] + other
        Duration.new(value + other, @parts.merge(seconds: seconds))
      end
    end
Compares one Duration with another or a Numeric to this Duration.
 Numeric values are treated as seconds.
 Adds another Duration or a Numeric to this Duration. Numeric values
 are treated as seconds.