The latest step in inversion controversy involving U.S. publicly traded corporations is the upcoming merger between pharmaceutical giants, Pfizer and Allergan, in a stock transaction estimated to be worth $160 billion. Kenneth Lobo and Stanley C. Ruchelman look at recent I.R.S. countermeasures attacking cross-border mergers that the I.R.S. views as inversions. Among other measures, rules are announced to limit planning alternatives using check-the-box entities to stuff assets into an acquirer without exposing those assets to tax in the jurisdiction of residence of the acquirer and use of parent-company stock as the consideration for the acquisition.