pt::grammar::peg
    start := (eos)
    rules
        eos :: <value> :=
            x
                (ex)
                *
                    x
                        +
                            <space>
                        (ex)
        ex :: <value> :=
            x
                +
                    x
                        !
                            (X)
                        /
                            'x'
                            'z'
                ?
                    (X)
        X :: <value> :=
            'x'
