@stream.write_with_handler(disco) do |result|
…
  # iterate thru disco results and wait until all collected
  EM::Iterator.new(result.items).map proc{ |c, it_disco|
…
    @stream.write_with_handler(info) do |reply|
…
      # iterate thru discoinfo results and wait until all collected,
      # then switch the parent’s iterator
      EM::Iterator.new(reply.features).map proc{ |f, it_info|
        it_info.return …
      }, proc{ |comps|
        # one more disco item was utilized
        it_disco.return …
      }
…             
    end
…
  }, proc{ |compss|
    # yielding
…
  }
end