Rules may span multiple lines,
but if we're still unbalanced at the
end of the file, something is wrong.

I write unbalanced rules depressingly often.

Change-Id: Ibd04aa06539e2a0ffef73bb665febf3542fd11f1
Reviewed-on: https://go-review.googlesource.com/12710Reviewed-by: Keith Randall <khr@golang.org>