"Pull-ad" clauses are provisions in a contract between an advertiser and a publication that give the advertiser wiggle room to remove its ads from an issue at the last minute or move them elsewhere in the publication so they don't run alongside news of major catastrophes, or even against pages containing stories about itself.
That revelation, which has been submitted for publication to Physical Review Letters, leaves no room for doubt: B-bars turn into B-minuses far faster than B-minuses turn into B-bars.