addPtsToSelection (indexes) {
    if (!Array.isArray(indexes)) { indexes = [indexes] }
    indexes.forEach((index) => {
      const seg = this.segs[index]
      if (seg.ptgrip && !this.selected_pts.includes(index) && index >= 0) {
        this.selected_pts.push(index)
      }
    })
    this.selected_pts.sort()
    let i = this.selected_pts.length
    const grips = []
    grips.length = i
    // Loop through points to be selected and highlight each
    while (i--) {
      const pt = this.selected_pts[i]
      const seg = this.segs[pt]
      seg.select(true)
      grips[i] = seg.ptgrip
    }

    const closedSubpath = Path.subpathIsClosed(this.selected_pts[0])
    svgCanvas.addPtsToSelection({ grips, closedSubpath })
  }
@param {Integer|Integer[]} indexes
@returns {void}