specifying merge base

"bzr help merge" says in the description

  By default, bzr will try to merge in all new work from the other
  branch, automatically determining an appropriate base. If this
  fails, you may need to give an explicit base.

However, it fails to ever tell you how to specify the merge base. I'm guessing maybe the -d argument?

The situation this question arose from is as follows. I have some really funky history I'm trying to import manually from a svn repo. I want to combine two different trees with history. I tried doing a "join" command, found out that's not supported very well. So instead I tried branching from tree A -r 0 to make tree B, in which I add my second project's history, and then merge trees A and B. But apparently -r 0 doesn't count as a revision for the purposes of determining common ancestors.