cc @billylanchantin

Are you able to use Explorer.Series.filter inside Explorer.DataFrame.summarise ?

I currently have some code that looks like this, but its giving me a compiler error about (ArgumentError) expected a variable to be given to var!, got: Explorer.DataFrame.pull( var!(df, Explorer.Query), :df )

    data_frame
    |> DataFrame.mutate(
      foo_id:
        if result in ["alice", "bob", "carol", "dave"] do
          bar_id
        else
          nil
        end
    )
    |> DataFrame.group_by(["some_idx"])
    |> DataFrame.summarise(
      baz:
        if count(foo_id) == 0 do
          "other/none"
        else
          first(filter(foo_id, is_not_nil(_)))
        end
    )