That would be no more intrusive than the current American rule stopping any one bank gaining more than a 10% market share of deposits by acquisition, a limit already touched by Bank of America.

ECONOMIST: The limits to size

In its most extreme interpretation, the rule would require a company that hires a labor-law firm to evaluate a potential acquisition with unionized operations, say, to report the relationship even if that blows its cover on secret due diligence activities.

FORBES: New Labor Dept. Rule Would Require Employers To Out Their Own Lawyers