tag signedness {
    signed;
    unsigned;
}

tag caseness {
    case_upper;
    case_lower;
}

tag ty {
    ty_bool;
    ty_str;
    ty_char;
    ty_int(signedness);
    ty_bits;
    ty_hex(caseness);
    // FIXME: More types
}

tag flag {
    flag_left_justify;
    flag_left_zero_pad;
    flag_left_space_pad;
    flag_plus_if_positive;
    flag_alternate;
}

tag count {
    count_is(int);
    count_is_param(int);
    count_is_next_param;
    count_implied;
}

// A formatted conversion from an expression to a string
type conv = rec(option.t[int] param,
                vec[flag] flags,
                count width,
                count precision,
                ty ty);

// A fragment of the output sequence
tag piece {
    piece_string(str);
    piece_conv(conv);
}