@Override
  public void extractFeatures(List regions) throws FeatureExtractorException {
    for (Region r : (List<Region>) regions) {
      if (r.hasAttribute("f0__tilt_amp") && r.hasAttribute("f0__tilt_dur") && r.hasAttribute("I__tilt_amp") &&
          r.hasAttribute("I__tilt_dur")) {
        r.setAttribute("skew_amp",
            ((Double) r.getAttribute("f0__tilt_amp")) - ((Double) r.getAttribute("I__tilt_amp")));
        r.setAttribute("skew_dur",
            ((Double) r.getAttribute("f0__tilt_dur")) - ((Double) r.getAttribute("I__tilt_dur")));
      }
    }
  }