docs: def smooth(polylines):
      """
      smooth every polyline using spline interpolation
      """
      for c in polylines:
          if len(c) < 9:
              # smoothing wouldn't make sense here
              continue
          x = c[:, 0]
          y = c[:, 1]
  
          t = np.arange(x.shape[0], dtype=float)
          t /= t[-1]
          x = UnivariateSpline(t, x)(t)
          y = UnivariateSpline(t, y)(t)
          c[:, 0] = x
          c[:, 1] = y