Code snippets to add to the book

the !!nm := val notation comes from rlang.

!! is quasiquotation, so you can assign to the variable named by nm, not 
to nm itself.

:= is an rlang operator that basically works like = but is less fussy, 
so you can use !!.

https://rlang.r-lib.org/reference/quasiquotation.html

https://github.com/rstudio/cheatsheets/blob/master/tidyeval.pdf
