anchor.diff:  %% for tests
   -export([init_stream/1]).
   
  +-type observable_doc() :: file_meta | file_location | times | custom_metadata.
  +-type filters() :: [observable_doc()].
  +
   -record(change_req, {
  -    record :: file_meta | file_location | times | custom_metadata,
  +    record :: observable_doc(),
       always = false :: boolean(),
       fields = [] :: [binary()],
       exists = [] :: [binary()]
   }).
  -
   -type change_req() :: #change_req{}.
   
   -define(DEFAULT_TIMEOUT, <<"infinity">>).
  
positive.code: %%% whether information about this record should be send always or
  %%% on this record changes only (`always` boolean flag with default value
  %%% being 'false' - not sending information on other docs changes).
  %%%
  %%% <record>:
  %%%     [fields: <fields>]
  
negative.code: %%% Handler for streaming changes happening to `file_meta`, `file_location`,
  %%% `times` or `custom_metadata` in scope of given space.
  %%%
  %%% Possible fields to observe are shown below.
  %%%
  %%% fileMeta:
  %%%     - name
  