> > > > MatProjectDirichlet() would be useful here, too: it would allow the > user to inject the right modifications to the near nullspace. >
I'm not sure this is a good abstraction. These are coordinate transformations. Here you just want to know if the value should be zero but in general you want a real transformation. If the user rotates the local coordinate system to match the BC and zeros out row(s) then there will be a dummy equation and you should, in theory, rotate the null space, and the dumb MatProjectDirichlet do its job. If the user _then_ rotates the operator back (what you initially suggested I think) then I don't think you need to do anything, now that I think about it.
