secure/precis: implementation of context rules only lookback is supported. Lookahead (except for the Katakana dot rule) is still forthcoming. Change-Id: Idf285da486d954597563f987ba6e75fc99f5cb22 Reviewed-on: https://go-review.googlesource.com/23805 Reviewed-by: Nigel Tao <nigeltao@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>