The epistemic closure is that:

 If S knows (if p then q) then (If S knows p then S knows q).

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In Nozick's  Truth-Tracking Analysis

  S knows p if and only if

 - p is true
 - S believes that p
 - If p were false, S would not believe that p
 - If p were true, S would believe that p

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The counter model here I mean something might includes

 - W (a nonempty set of worlds),

 - R (a relation over W), and 

 - I(SxW) (where I is the interpretation function I:(s,w)->{0,1},
where S is the set of sentense letters.
This should tells us whether a sentense letter is true or not true in a world.)

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To show the epistemic closure is not valid under Nozick's system, we simply find a counter model with world w s.t. the negation of epistemic closure is true in this world.

I tried to do this by myself, but I didn't figure out Nozick's semantics by reading his *philosophical explansion*

for example

 - How did Nozick define the valuation of the belief operator
 - For the relation of worlds, what rules they must obey? (are they transitive, or symmetric etc.)
 - What's the valuation of Nozick's counterfactual conditionals 

(I know Nozick's system of counterfactual conditionals is different from Stalnaker and Lewis.)

What Nozick said on page 173 is that
>This point is brought out especially clearly in recent 'possible-worlds' accounts of subjunctives: The subjunctive is true when (roughly) in all those worlds in which p holds true that are closest to the actual world, q also is true (Examine those worlds in which p holds true closest to the actual world, and see if q holds true in all these.)

To me, this really sounds like Stalnaker's account of subjunctive conditionals:
>That p subjunctive implies q is true in w if and only if for all worlds x, [if p is true in x and for all world y (if p true in y then x sees y under relation Rw)], then x true in q.

Could someone explain Nozick's semantics to me in details?

Any help or suggestion would be appreciated.