function hookTension(ratio) {
  return glyph => glyph.map(contour => {
    const result = JSON.parse(JSON.stringify(contour));
    result.forEach((pt1, i) => {
      const pt2 = result[mod(i+1, result.length)],
            pt3 = result[mod(i+2, result.length)],
            pt4 = result[mod(i+3, result.length)],
            pt5 = result[mod(i+4, result.length)],
            pt6 = result[mod(i+5, result.length)];
      if (isHook(pt1, pt2, pt3, pt4, pt5, pt6)) {
        const cx = (pt3.x + pt4.x)/2;
        pt3.x = pt3.x * (1-ratio) + cx * ratio;
        pt4.x = pt4.x * (1-ratio) + cx * ratio;
      }
    });
    return result;
  });
}
Tension of the horizonal hook strokes
@param { number } ratio 
@returns { PostFilter }