code: function checkCorrectChoiceParam(that, choice) {
          if ('number' === typeof choice) choice = '' + choice;
          if ('string' !== typeof choice) {
              throw new TypeError('ChoiceTable.setCorrectChoice: each choice ' +
                                  'must be number or string. Found: ' + choice);
          }
          if ('undefined' === typeof that.choicesValues[choice]) {
  
              throw new TypeError('ChoiceTable.setCorrectChoice: choice ' +
                                  'not found: ' + choice);
          }
          return choice;
      }
docs: ## Helper methods. 
  ### checkCorrectChoiceParam
  
  Checks the input parameters of method ChoiceTable.setCorrectChoice
  
  The function transforms numbers into string, because then the checking
  is done with strings (they are serialized in the id property of tds).
  
  If `ChoiceTable.selectMultiple` is set, the function checks each
  value of the array separately.
  
  @param {ChoiceTable} that This instance
  @param {string|number} An already existing value of a choice
  
  @return {string} The checked choice