1: -((-A--B) V(B--A)) assumption 2: assumption 3: assumption 4: assumption 5: hyp 2 6: intro 4-5 7: v intro 6 8: elim 7,1 9: - intro 3-8 10: intro 2-9 11: v intro 10 12: - elim 11,1 13: (A-B) V(B--A) contra (classical) 1-12 Select one or more: line 6 and line 10 contains the same formula line 9: -A line 9: A line 12: 1 line 8:1 line 10: - A B Line 7 and line 11 contain the same formula line 10: B-A