code: public function asBoolean($value)
      {
          if ($value === null) {
              return $this->nullDisplay;
          }
  
          return $value ? $this->booleanFormat[1] : $this->booleanFormat[0];
      }
docs: Formats the value as a boolean.
  @param mixed $value the value to be formatted.
  @return string the formatted result.
  @see booleanFormat