The idea comes straight from
aes in ggplot2. That provides a way to map columns of a dataset to features of graphic.
Here we are mapping columns into a function so we can use standard names inside that function. This function variant
expects unquoted string arguments, rather than the quoted version
Unquoted, comma-seperated column mappings
A mapping looks like: <column_to_be_created> = <existing column>
aes_d(group = class) #> $group #> class #>