Method maybe

src/stone/Parser.java:366–371

(Parser p)

        return this;
    }
    public Parser maybe(Parser p) {
        Parser p2 = new Parser(p);
        p2.reset();
        elements.add(new OrTree(new Parser[] { p, p2 }));
        return this;
    }
    public Parser option(Parser p) {
        elements.add(new Repeat(p, true));
        return this;