macro_rules! expand_custom_vec_vec {
($(impl Custom for Vec<Vec<$fname:ident>> ($func:ident, $extract:ident, $default:expr, $to_json_array:ident))*) => { ... };
}
macro_rules! expand_custom_vec_vec {
($(impl Custom for Vec<Vec<$fname:ident>> ($func:ident, $extract:ident, $default:expr, $to_json_array:ident))*) => { ... };
}