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function onEdit() {
  // Re: https://productforums.google.com/d/topic/docs/gnrD6_XtZT0/discussion
  //
  // This script prevents cells from being updated. When a user edits a cell on any sheet,
  // it is checked against the same cell on a helper sheet, and:
  //
  //   - if the value on the helper sheet is empty, the new value is stored on both sheets
  //   - if the value on the helper sheet is not empty, it is copied back to the cell on
  //     the source sheet, undoing the change
  //