def without(*features)
      features = setify(*features)
      self.class.new Hash[@sets.select {|key, val| !features.subset?(key)}]
    end
def without(*features)
      features = setify(*features)
      self.class.new Hash[@sets.select {|key, val| !features.subset?(key)}]
    end
Return an instance of Sounds whose sets exclude any of the given
 features.