standby, pg_basebackup and last xlog file

Hello!

Is there any reason why pg_basebackup has limitation in an online backup from the standby: "The backup history file is not created in the database cluster backed up."
?

So i can't get last xlog file needed to restore :(

Think i can use -x option for getting last xlog file, but i would like to minimize size of resulting backup. // i also get all WALs by archive_command, so there is no reason to get two copies of each wal during basebackup.

Also maybe i can use something like ( pg_last_xlog_replay_location() + 1 ) after pg_basebackup finished.

Does anybody know true way to getting last xlog file in case of applying pg_basebackup to standby
?
How pg_basebackup gets last xlog file in case of standby and -x option
?