struct DataTypeDeclarationField :
    g::And<id, colon, Expression, g::Opt<g::And<equal, Expression>>>
{
    Field make(DeclarationScopeParser& parser)
    {
        Box<ast::Expression> init;
        if ( auto c = factor<3>().capture() )
            init = c->factor<1>().make(parser);

        return {
            ast::Symbol(factor<0>().token()),
            flattenConstraints(factor<2>().make(parser)),
            std::move(init)
        };
    }
};


Field parse_DataTypeDeclarationField(DeclarationScopeParser& parser) {
    auto id = parse_id(parser);
    parse_colon(parser);
    auto expression = parse_Expression(parser);
    ast::Expression rhs = nullptr;
    if (try_parse_equal(parser)) {
        rhs = parse_Expression(parser);
    }
    return {
        ast::Symbol(id.token()),
        flattenConstraints(expression),
        std::move(rhs)
    };
}