'three-year-look-back rule' issued by the Treasury

The new rule does not allow stock accumulated through a foreign company’s U.S. deals in the last three years to count towards the book value needed to meet the inversion threshold.
This weighed on Allergan heavily because of its significant deals in this timeframe. These include the $66 billion merger of Allergan and Actavis Plc, the $25 billion purchase of Forest Laboratories and the $5 billion takeover of Warner Chilcott.
“The serial acquisition portion of the regulations will cause Pfizer to be treated as an ‘expatriated entity’ (under the terms of its existing deal with Allergan),” Robert Willens, a corporate tax and accounting analyst, wrote in a note.