motion/duration.rb
def +(other)
      if Duration === other
        Duration.new(value + other.value, @parts + other.parts)
      else
        Duration.new(value + other, @parts + [[:seconds, other]])
      end
    end
def +(other)
      if Duration === other
        Duration.new(value + other.value, @parts + other.parts)
      else
        Duration.new(value + other, @parts + [[:seconds, other]])
      end
    end
Adds another Duration or a Numeric to this Duration. Numeric values
 are treated as seconds.