(May 21, 2018, 2:26 PM EDT) -- WILMINGTON, Del. — A federal bankruptcy judge in Delaware erred in using the interest rate provided for in the Chapter 11 plan of W.R. Grace & Co. in calculating a tax refund owed by the Internal Revenue Service because the agency’s recoupment rights are not “claims” under the U.S. Bankruptcy Code, the IRS says in its May 2 opening brief on appeal to the district court (In re: W.R. Grace & Co., et al., United States v. W.R. Grace & Co., et al., No. 17-cv-1588, D. Del.)....