public Parser(File f, IdentifierTable t) throws Exception {
        idTable = t;
        scan = new Scanner(f, idTable);
        env = new Environment();

        // add builtin function names
        if(inputNum == null) {
            inputNum = idTable.addToTable("InputNum");
        }
        if(outputNum == null) {
            outputNum = idTable.addToTable("OutputNum");
        }
        if(outputNewLine == null) {
            outputNewLine = idTable.addToTable("OutputNewLine");
        }
    }