src/HOL/SPARK/Examples/Sqrt/Sqrt.ads

clarified NEWS: document_files are officially required since Isabelle2014, but the absence was tolerated as legacy feature;

package Sqrt is

   function Isqrt(N: Natural) return Natural;
   --# return R => R * R <= N and (R + 1) * (R + 1) > N;

end Sqrt;