code: function Words(superwords, classwords) {
  	this.classwords = getwords(classwords, ['extend', 'mixin']);
  	this.superwords = getwords(superwords, ['this._super']).map(function dot(
  		word
  	) {
  		return word + '.';
  	});
  }
docs: Tracking word(clusters) that subclasses something
  and other words that seem to call a super method.
  @param @optional {boolean|string|Array<string>} superwords
  @param @optional {boolean|string|Array<string>} classwords