public boolean consumeBoolean() throws ParseException {
      if (currentToken.equals("true") ||
          currentToken.equals("t") ||
          currentToken.equals("1")) {
        nextToken();
        return true;
      } else if (currentToken.equals("false") ||
                 currentToken.equals("f") ||
                 currentToken.equals("0")) {
        nextToken();
        return false;
      } else {
        throw parseException("Expected \"true\" or \"false\".");
      }
    }