tcSubsumes

Hi Simon, Could you take a look at TcSimplify.tcSubsumes? This relatively new function pushes the TcLevel when calling tcSubType, but then doesn't set the TcLevel when calling the solver. Inexplicably, this sometimes works -- even passing the testsuite. Somehow, my work caused this to fail. I fixed it by setting the TcLevel before calling the solver. Two questions: 1. Is this a reasonable fix? 2. (optional) How is it possible that this didn't fail drastically often? Note (to author / reviewers of this function): my astonishment isn't that this error crept into the codebase -- I've done similar many many times. My astonishment is how the testsuite didn't find this error forthwith. Thanks, Richard