fun pbor p q r = case (p, q, r) of
                     (true, true, true) => false |
                     (true, true, false) => true |
                     (true, false, true) => false |
                     (true, false, false) => false |
                     (false, true, true) => false |
                     (false, true, false) => true |
                     (false, false, true) => true |
                     (false, false, false) => false;