Research adding `meta` classes

It might be useful to define class such as `list` of `data.frame` etc:

```r
`list<data.table>` <- setClass(
    "list<data.table>",
    slots = list(
        .data = "list"
    ),
    prototype = list(
        .data = list()
    ),
    validity = function(object) {
        unique(sapply(object@.data, data.table::is.data.table)) != c("data.table" "data.frame")
    }
)
```