Finding the relative path of a file from a dir

[Suresh Jeevanandam]

Hi,
	a = '/home/suresh/doc/html/a1/'
	b = '/home/suresh/doc/'

I am looking for a standard function which will return the location of b 
relative to a i.e. '../..'

	I have gone through the os and os.path modules, but could not find any 
function of use. Should I write my own?

Thanks in advance,
Suresh