+-spec map_error(t(A, B), fun((B) -> (C))) ->
+ t(A, C).
+map_error({ok, _}=Ok, _) ->
+ Ok;
+map_error({error, Reason}, F) ->
+ {error, F(Reason)}.
+
+-spec tag_error(t(A, Reason), Tag) ->
+ t(A, {Tag, Reason}).
+tag_error({ok, _}=Ok, _) ->
+ Ok;
+tag_error({error, Reason}, Tag) ->
+ {error, {Tag, Reason}}.
+