doc-src/Tutorial/Misc/NatSum.ML

removed prs / prs_fn (broken, because it did not include \n in its
semantics, forcing writeln to add one uncoditionally);
replaced prs_fn by writeln fn;

Goal "sum n + sum n = n*(Suc n)";
by(induct_tac "n" 1);
by(Auto_tac);