Meinongian arguments

Problem (basically due to Oppy): Take a particular unicorn. This is defined by a complete compatible collection of properties. Now, replace non-existence with existence. We either get an incompatible or a compatible collection. If incompatible, unicorns are impossible, and I shall suppose they're possible. Suppose compatible. Then they are not complete--for if they were complete, then we'd have a complete compatible collection of properties that includes existence, and hence unicorns would exist.