code: @Test public void nested() throws ScanException {
    String input="a${b${c}}";
    Tokenizer tokenizer=new Tokenizer(input);
    List<Token> tokenList=tokenizer.tokenize();
    witnessList.add(new Token(Token.Type.LITERAL,"a"));
    witnessList.add(Token.START_TOKEN);
    witnessList.add(new Token(Token.Type.LITERAL,"b"));
    witnessList.add(Token.START_TOKEN);
    witnessList.add(new Token(Token.Type.LITERAL,"c"));
    witnessList.add(Token.CURLY_RIGHT_TOKEN);
    witnessList.add(Token.CURLY_RIGHT_TOKEN);
    assertEquals(witnessList,tokenList);
  }
  