slice(points, start, end) {
    const pts = points.slice(start, end)

    if (start) {
      // Add additional 'C' points
      if (end - start > 1 && pts[1].length < 6) {
        const n = pts[0].length

        pts[1] = [
          pts[0][n - 2] * 2 - pts[0][n - 4],
          pts[0][n - 1] * 2 - pts[0][n - 3],
        ].concat(pts[1])
      }
      // Remove control points for 'M'
      pts[0] = pts[0].slice(-2)
    }

    return pts
  }
Slice out a segment of 'points'
@param {Array} points
@param {Number} start
@param {Number} end
@returns {Array}