/**
 * Extract formula from a cell
 *
 * @param {reference} a cell reference from which to extract the formula
 * @return a string representation of the formula in {reference}
 * @customfunction
 */

function CELLFORMULA(reference) {
  var ss = SpreadsheetApp;
  var sheet = ss.getActiveSheet();
  var formula = ss.getActiveRange().getFormula();
  var args = formula.match(/=\w+\((.*)\)/i);
  try {
    var range = sheet.getRange(args[1]);
  }
  catch(e) {
    throw new Error(args[1] + ' is not a valid range');
  }
  return range.getFormula();
}