19.4.7 nonstandard ast

[TylerGrantSmith]

https://github.com/hadley/adv-r/blob/f6c437a8baff551ffff6b602c38f1210c615ad17/Quotation.Rmd#L578-L585

This isn't entirely true.

lobstr::ast(`+`(1, `+`(2,3)))
#> o-`+` 
#> +-1 
#> \-o-`+` 
#>   +-2 
#>   \-3

^{Created on 2020-06-03 by the reprex package (v0.3.0)}