What's the worst thing that happened to Donald Trump this week? It was NOT Director Comey's testimony. Rather, it must be the late Friday news that Robert Mueller has hired Michael Dreeben, on a part-time basis, to help with his investigation. Dreeben, a deputy in the Office of the Solicitor General, has argued more than 100 cases before the Supreme Court. His specialty has, for the last 20 years, been criminal matters and he has an encyclopedic knowledge of criminal law. I once saw him argue a Supreme Court matter without a single note. In short, he is quite possibly the best criminal appellate lawyer in America (at least on the government's side). That Mueller has sought his assistance attests both to the seriousness of his effort and the depth of the intellectual bench he is building.