awk's seen idiom

I wanted to extract distinct values of a particular column in a csv file, and just found awk has a concise idiom to do just that. Say you want to operate on the 2nd column, hence $2 in the following snippet. This is all it takes:

awk -F',' '!seen[$2]++' a.csv

seen is merely an example. You may use (at least) anything from the [A-Za-z_]+ domain.