Pfizer and Allergan have cancelled their $150 billion merger agreement, which would have created the largest pharma giant in the world. The agreement was terminated because of a recent change in U.S. tax rules. NPR’s Jim Zarroli explained the policy shift: “The Treasury Department on Monday introduced rules aimed at reducing the incentives for companies to carry out inversions. That’s a controversial practice in which a U.S. company merges with a firm in a foreign coun...