def <<(*args)
      args.to_a.flatten.each do |feature|
        features.subtract Features.set(feature).to_a
        add! feature
      end
      self
    end
def <<(*args)
      args.to_a.flatten.each do |feature|
        features.subtract Features.set(feature).to_a
        add! feature
      end
      self
    end
Add a feature, replacing either the place or manner of articulation,
 or the height or backness. Returns self.