How to get counts of an array_agg

[mhkeller]

What's the best way to get the count of unique items from an array_agg?

const t = aq.table({
  group:  ["a", "a", "a", "b", "b", "b", "b", "b", "c", "c", ],
  answer: ["n", "y", "y", "y", "n", "n", "n", "y", "y", "n"]
}).groupby('group')
  .derive({
    answers: d => op.array_agg(d.answer)
  })
  .derive({
    answer_counts: ???
  })

I now want some data like this for each group fo `answer_counts

{
  n: 1,
  y: 2
}

Or could be an array

[ { key: 'n', count: 1 }, { key: 'y', count: 2 }]

Or a string like n_1|y_2 so I can write this out to a flat csv