Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers.
Many are able to help even without being deeply knowledgable without NLP. They outnumber NLP experts but are unlikely to address a question without a reprex.
This almost makes it, but it missing an essential ingredient, the data represented by the doc argument.
Without it, the most I can help with is the bracketing operator.
Basically, it selects parts of a list. For example
head(mtcars)
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
#> Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
#> Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
#> Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
#> Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
#> Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1
head(mtcars[1])
#> mpg
#> Mazda RX4 21.0
#> Mazda RX4 Wag 21.0
#> Datsun 710 22.8
#> Hornet 4 Drive 21.4
#> Hornet Sportabout 18.7
#> Valiant 18.1
mtcars[1,1]
#> [1] 21
Created on 2020-04-06 by the reprex package (v0.3.0)
Lists are also objects that can contain other objects, nested within them. And those objects, also.
So for a list of lists the double brackets address the list itself rather than the list's contents.