Question 1209011
<pre>
1. W  ⊃ Z
2. W  v Z  / ∴    Z

                |3.  ~Z      assumption for indirect proof
                |4.  Z v W       2, commutation
                |5.  W         4,3, disjunctive syllogism
                |6.  Z         1,5, modus ponens    
                |7. Z & ~Z     6,3, conjunction 
8. Z      lines 3-7 for indirect proof

Edwin</pre>