revert working tree using bzrlib

[Zunbeltz]

Hi,

I started to use bzrlib from an script.
I got thw WorkingTree object and I would like to revert the working tree
back and ford; like with the command line
bzr revert -r3

WorkingTree has the following method
@needs_tree_write_lock
def revert(self, filenames=None, old_tree=None, backups=True, pb=None,
report_changes=False): 

But it is not documented, and I do not figure out where the revno should go.

TIA

Zunbeltz