Hi Julian & interested parties, I had this issue popping up and no clear idea where to put it in the wiki - so I'm posting it here.
Since symmetric merge has more freedom in choosing base and merge order, the meaning of "base", "mine", "theirs" etc. might become ambiguous if they are being derived automatically from the diff3 being executed. I simply want to note that we might need to explicitly tell the wc which input has which meaning for a given merge step. -- Stefan^2.