clarified NEWS: document_files are officially required since Isabelle2014, but the absence was tolerated as legacy feature;
package Simple_Greatest_Common_Divisor
is
--# function Gcd (A, B : Natural) return Natural;
procedure G_C_D (M, N : in Natural; G : out Natural);
--# derives G from M, N;
--# pre N > 0;
--# post G = Gcd (M, N);
end Simple_Greatest_Common_Divisor;