Usefulness of the "not in" operator

Sat Oct 8 06:42:58 EDT 2011

Python provides
 -- the not operator, meaning logical negation
 -- the in operator, meaning membership
On the other hand, Python provides the not in operator meaning 
non-membership. However, it seems we can reformulate any "not in" 
expression using only "not" and "in" operation. For instance
 >>> 'th' not in "python"
False 
 >>> not ('th' in "python")
False
 >>>
So what is the usefulness of the "not in" operator ? Recall what Zen of 
Python tells
There should be one-- and preferably only one --obvious way to do it.