Pickaxe usage question -- only matching on added string

Documentation for diffcore-pickaxe (in Documentation/diffcore.txt) says:

When diffcore-pickaxe is in use, it checks if there are filepairs whose "original" side has the specified string and whose "result" side does not. Such a filepair represents "the string appeared in this changeset". It also checks for the opposite case that loses the specified string.

Now, is there a way to get diffcore to match only on 'added' (or on 'removed', for that matter)? I am tring to identify commtis that added patches to a project, and I seem to be getting matches that add and remove when I do:

   git-whatchanged -p -C -S"\t" master

cheers,

martin