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Figure 3. Completed tableau for (a Vv b) >a.  form a contradiction. Similarly, node 3 is an ancestor of node 8 and when read conjunc- tively, trivially forms a contradiction. All branches are contradictory and closed, therefore the original formula is valid.

Figure 3 Completed tableau for (a Vv b) >a. form a contradiction. Similarly, node 3 is an ancestor of node 8 and when read conjunc- tively, trivially forms a contradiction. All branches are contradictory and closed, therefore the original formula is valid.