code: private String getIndentString() {
      assert enabled;
      if (indentString == null) {
        for (int i = indentStrings.size(); i <= indentLevel; i++) {
          indentStrings.add(indentStrings.get(i - 1) + INDENT_STR_ONE_LEVEL);
        }
        indentString = indentStrings.get(indentLevel);
      }
      return indentString;
    }
docs: Return the current indentation string.