newbie question: using bzrlib api

[Rohit Nayak]

I am integrating bazaar into a python web application. This is what I
am using currently to add a file:

from bzrlib import workingtree
class bzrVCS:
    def __init__(self, dir):
        self.wt = workingtree.WorkingTree.open(REPODIR)
    def add(self, path):
        fileList = [path]
        self.wt.add(fileList)
    def commit(self, path, label=""):
        fileList = [path]
        self.wt.commit(message=label, specific_files=fileList)

This works fine.

However I am unable to figure out what the api is to
      * get the list of previous versions (and related metadata: time
of commit, label)
      * get the contents of one of the previous versions

The bzr version is 1.2.0.

Any help or pointers to relevant posts/docs would be appreciated!
-- 
Thanks!
Rohit