For now, limited to a few repetitive boolean rules where the win is
substantial (4+ variants).

Change-Id: I67bce0d356ca7d71a0f15ff98551fe2caff8abf9
Reviewed-on: https://go-review.googlesource.com/95535
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>