Method setCheckpoint

modules/core/EditSystem.js:568–577

* setCheckpoint * This saves the `history` and `index` as a "checkpoint" that we can return to later. * If the given checkpointID exists, it will be overwritten. * @param {string} checkpointID - A string to identify the checkpoint

(checkpointID)

   * @param  {string}  checkpointID - A string to identify the checkpoint
   */
  setCheckpoint(checkpointID) {
    if (!checkpointID) return;
    d3_select(document).interrupt('editTransition');    // complete any transition already in progress

    // Save a shallow copy of history, in case user undos away the edit that `_index` points to.
    this._checkpoints.set(checkpointID, {
      history: this._history.slice(),  // shallow copy
      index: this._index
    });
  }


  /**