- issue405: The goal violates a mutex, which the translator should
  detect and produce a trivial unsolvable problem. The search code
  should then report the problem as unsolvable. (It may be the case
  that this example is a bit fragile because it's based on
  Blocksworld, and I guess there's no guarantee which of the two major
  Blocksworld encodings we get. I think only one of them will detect
  that there is a mutex violation.)
