function onEdit() {
  // Re: /docs/forum/AAAABuH1jm0gnrD6_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
  //
  // This in effect protects sheets in a "write once" manner, albeit with lots of limitations.
  // The script does *not* provide protection against edits by a determined user.
  // For example, selecting all cells and pressing Delete lets any user erase many cells at once.
  // The values that were deleted from the cells will however be preserved on the helper sheet
  // and will reappear when an individual cell is edited again.
  //
  // Helper sheets are created automatically when an edit is first made, one helper sheet
  // per source sheet. For a source sheet named "Sheet1", the helper sheet is "Sheet1_helper".
  // Helper sheets are automatically hidden when created to not clutter the display, but
  // they can be uhidden by any user with "can edit" rights to the spreadsheet.
  // Users with edit rights can also disable this script at will.
  //
  // To change a value that was entered previously, empty the corresponding cell on the helper sheet,
  // then edit the cell on the source sheet.
  // When you rename a source sheet, remember to rename the helper sheet as well.
  // Choose "View > Hidden sheets" to show the helper sheet, then rename it using the pop-up
  // menu at the sheet's tab at the tab bar at bottom of the browser window.
  //
  // To take this script into use:
  //
  //  - take a backup of your spreadsheet through File > Make a copy
  //  - select all the text in this script, starting at the "function onEdit()" line and ending at the last "}"
  //  - copy the script to the clipboard with Control+C
  //  - open the spreadsheet where you want to use the function
  //  - choose Tools > Script editor > Blank (this opens a new tab in the browser)
  //  - press Control+A followed by Control+V to paste the script in
  //  - press Control+S to save the script
  //  - close the script editor tab and go back to the spreadsheet tab
  //
  // The script will from then on watch updates on all the sheets and only allow edits
  // when the cell is empty to start with.
  //
  // Note that the script only protects _values_ rather than _formulas_.
  // To protect formulas, use Data > Named and protected ranges.
  //
  // If your sheets that you would like to protect already have data on them, create helper
  // sheets manually by choosing the Duplicate command from the sheet's tab menu at the tab bar
  // at the bottom of the browser window. Rename the new sheet so that "Copy of Sheet1" becomes
  // "Sheet1_helper".
  //
  // The range where edits are of this "write once" type can be limited by changing the values
  // assigned to the firstDataRow, lastDataRow, firstDataColumn and lastDataColumn variables below.
  // The range defined by these values is global and will apply to all the sheets the same.
  //
  // You can exclude some sheets from being watched by putting them on the freeToEditSheetNames
  // list. See below for more info.


  // modify these variables per your requirements

  // define the range where edits are "write once"
  // to watch only the range A1:D100, define rows as 1,100 and columns as 1,4
  // to watch only the range M20:V30, define rows as 20,30 and columns as 13,22
  var firstDataRow = 1; // only take into account edits on or below this row
  var lastDataRow = 999; // only take into account edits on or above this row
  var firstDataColumn = 1; // only take into account edits on or to the right of this column
  var lastDataColumn = 999; // only take into account edits on or to the left of this column

  // naming pattern for sheets where values are copied for later checking
  var helperSheetNameSuffix = "_helper";

  // sheets that are free to edit with no protection
  var freeToEditSheetNames = ["Free to edit 1", "Free to edit 2", helperSheetNameSuffix + "$"];
  // You can use regular expressions in sheet names. The match is not case-sensitive,
  // so "free.*edit" will match "free to edit", "Free Editing for Everyone",
  // "Sheet (free to edit)" and "Free edit playground".
  // Leave the last entry, helperSheetNameSuffix + "$", as it is to ensure that changes to a
  // helper sheet do not trigger the creation of another _helper_helper sheet.
  // See these sites for more info:
  //   - http://en.wikipedia.org/wiki/Regular_expression
  //   - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions


  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var masterSheet = ss.getActiveSheet();
  var masterSheetName = masterSheet.getName();
  var masterCell = masterSheet.getActiveCell();
  var sheetNameRegExp;

  // do not record edits on free to edit sheets
  for (var sheet in freeToEditSheetNames) {
    sheetNameRegExp = new RegExp(freeToEditSheetNames[sheet], "i");
    if (sheetNameRegExp.test(masterSheetName)) return;
  }

  // find helper sheet
  var helperSheetName = masterSheetName + helperSheetNameSuffix;
  var helperSheet = ss.getSheetByName(helperSheetName);
  if (helperSheet == null) { // helper sheet does not exist yet, create it as the last sheet in the spreadsheet
    helperSheet = ss.insertSheet(helperSheetName, ss.getNumSheets());
    Utilities.sleep(2000); // give time for the new sheet to render before going back
    ss.setActiveSheet(masterSheet);
    helperSheet.hideSheet();
    ss.setActiveRange(masterCell);
  }

  if (masterCell.getRow() < firstDataRow || masterCell.getColumn() < firstDataColumn ||
    masterCell.getRow() > lastDataRow || masterCell.getColumn() > lastDataColumn) return;

  var helperCell = helperSheet.getRange(masterCell.getA1Notation());
  var newValue = masterCell.getValue();
  var oldValue = helperCell.getValue();

  if (oldValue == "") {
    helperCell.setValue(newValue);
  } else {
    masterCell.setValue(oldValue);
  }
}