bug#5689: looking-back-p

[Stefan Monnier]

> Please include the missing looking-back-p

looking-back is a function that people should generally try to avoid,
because it's very costly and its behavior is not always as people
might expect.  So if anything, I'd rename it to
do-something-like-looking-at-but-going-backward and would definitely
refrain from providing a "-p" alternative that would make it sound like
"it's more lightweight" even tho it's actually even worse (tho
unnoticeably so).



        Stefan