public Node parse() throws ScanException {
    if (tokenList == null || tokenList.isEmpty())
      return null;
    return E();
  }
@Test
  public void literal() throws ScanException {
    Tokenizer tokenizer = new Tokenizer("abc");
    Parser parser = new Parser(tokenizer.tokenize());
    Node node = parser.parse();
    Node witness = new Node(Node.Type.LITERAL, "abc");
    assertEquals(witness, node);
  }