le.

(Fact)   C =&gt; A,   In the Current situation the Air is cool.

(Rule)   B =&gt; A,   Just Before it rains, the Air is cool.

(Case)   C =&gt; B,   The Current situation is just Before it rains.

2.  The final step is a Deduction that admits this Case
    to another Rule and so arrives at a novel Fact.

(Case)   C =&gt; B,   The Current situation is just Before it rains.

(Rule)   B =&gt; D,   Just Before it rains, a Dark cloud will appear.

(Fact)   C =&gt; D,   In the Current situation, a Dark cloud