Function for determining one XPath as subset of anothe

Given two simple XPaths, say:

1. //w

2. /x/y/z/w[@a = 'v']

As a human I can very easily tell without evaluating the expressions
that (2) will return a subset (or the same set) of the results that
(1) would return *should* they both be evaluated.

My goal here is given any two simple arbitrary XPaths expressed as
strings, and without evaluating them against a context, to determine
whether one would return a subset of the results of the other.

I wondered if there might be an algorithm or library that someone
already had or has written which might be able to give me the answer?

I realise that I can only probably cover a subset of XPath itself, but
it is only the path steps with predicates which I am interested in.

Ideally I am looking for something in Java.

-- 
Adam Retter

skype: adam.retter
tweet: adamretter
http://www.adamretter.org.uk