Suppose that $,,$ are well$-$formed formulas generated by some propositions. Now, we

know something about the world that $\{\}|=$ holds. Meaning that we don$$t know exactly about

which wffs $(,,)$ are true in our world nor any truth$-$value assignments about the propositions

$($model$).$ However, we know that the entailment must hold, $\{\}|=.$

In this case, show that $\{\}|=(->)$ holds. This may seem hard: but basically, what we$$re trying to

do is show that the first entailment rules out some possible worlds where the entailment doesn$$t hold.

Once we$$ve ruled out those worlds, we can show that the second entailment holds in the worlds that

are left. To do this, use a few steps $($show all of these$)$: