anchor.diff:        Error
     end.
   
  --spec find_one(pid(), colldb(), mongo:selector(), readprefs()) -> map().
  +-spec find_one(map(), colldb(), mongo:selector(), readprefs()) -> map().
   find_one(#{pool := Pool, server_type := ServerType, readPreference := RPrefs}, Coll, Selector, Options) ->
     Projector = mc_utils:get_value(projector, Options, []),
     Skip = mc_utils:get_value(skip, Options, 0),
  
positive.code: 
  %% @doc Returns projection of selected documents.
  %%      Empty projection [] means full projection.
  find(Pool, Coll, Selector) ->
    find(Pool, Coll, Selector, []).
  
  find(#{pool := Pool, server_type := ServerType, readPreference := RPrefs}, Coll, Selector, Options) ->
    Projector = mc_utils:get_value(projector, Options, []),
    Skip = mc_utils:get_value(skip, Options, 0),
  
negative.code: command(Pool, Command) ->
    command(Pool, Command, undefined).
  
  -spec command(map(), bson:document(), readprefs()) -> {boolean(), bson:document()} | {error, reason()}. % Action
  command(#{pool := Pool, server_type := ServerType, readPreference := RPrefs}, Command, Db) ->
    Q = #'query'{
      collection = {Db, <<"$cmd">>},
  