The option is actually Aptitude::ProblemResolver::PreserveManualScore; no hyphen. This is contrary to the documentation, which I've now fixed. The reason that this is not a very strong imperative is that it forces the program to try *very hard* to preserve the states of manual packages. This in turn means that it can be difficult to find any solution at all (indeed, I was able to quickly guide the search to a solution without setting this; with this option, it ran out of solutions almost right away).
You could instead set Aptitude::CmdLine::Request-Strictness to a large value. This just tries to preserve the states of packages listed on the command-line. It isn't set by default because I feel that the command-line and the visual interface should have identical behavior. Even this wasn't enough to make your preferred option come up first, although it was the third solution I got. Daniel
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