Mainly used for documenting the parser code.
Examples
library(tibble)
ta <- tibble(a = 1:2)
tb <- tibble(i = 1, j = nested_tibble(ta))
ta
#> # A tibble: 2 × 1
#> a
#> <int>
#> 1 1
#> 2 2
tb
#> # A tibble: 1 × 2
#> i j
#> <dbl> <list>
#> 1 1 <tibble [2 × 1]>