It seems very appealing to restrict Break so that it only works when
there's one word on the stack. Then we can pop that word, mark it as the
root, and continue.
However, results are suggesting it's nice to be able to predict Break
when the last word of the previous sentence is on the stack, and the
first word of the next sentence is at the buffer. This does make sense!
Consider that the last word is often a period or something --- a pretty
huge clue. We otherwise have to go out of our way to get that feature
in.
The really decisive thing is we have to handle upcoming sentence breaks
anyway, because we need to conform to preset SBD constraints. So, we may
as well let the parser predict the Break when it's at a stack/queue
position that is most revealing.
