This class manipulates punctuation in trees (used with training trees)
in the same manner that Collins manipulated punctuation in trees when
building his parsing model. This is the same punctuation that is
the punctuation ignored in the standard EvalB evaluation is promoted
as high in the tree as possible.
Does whatever one needs to do to a particular tree.
This routine is passed a whole Tree, and could itself
work recursively, but the canonical usage is to invoke this method
via the Tree.transform() method, which will apply the
transformer in a bottom-up manner to each local Tree,
and hence the implementation of TreeTransformer should
merely examine and change a local (one-level) Tree.