How to obtain the number of rows in each group using DataFramesMeta?

first-steps
#1
using DataFrames, DataFramesMeta

a = DataFrame(a = rand(1:10, 100))

@> begin
   a
   @by(:a, count = length(:a))
end

is there a more convenient function like R’s dplyr::n() which returns the number of rows? Or even, R’s data.table's .N?

#2

How about:

by(a, :a, nrow)
#3

Hoping to use DataFramesMeta code.