def record_ids(opts = {})
      opts = @opts.merge(opts)
      request_with_sets(opts) do |set_opts|
        client.list_identifiers(set_opts).full.lazy.flat_map(&:identifier)
      end
    end
def record_ids(opts = {})
      opts = @opts.merge(opts)
      request_with_sets(opts) do |set_opts|
        client.list_identifiers(set_opts).full.lazy.flat_map(&:identifier)
      end
    end
@param opts [Hash] options to pass through to client requests.
   Allowable options are specified in OAI::Const::Verbs. Currently :from,
   :until, :set, and :metadata_prefix.
   Additionally, you may pass an xpath string to `:id_path` specifying the
   location of the IDs.
 @see OAI::Client
 @see #expected_opts

 Sends ListIdentifier requests lazily.

 The following will only send requests to the endpoint until it
 has 1000 record ids:

     record_ids.take(1000)

 @param opts [Hash] opts to pass to OAI::Client
 @see #expected_opts