* HOL/Library: a collection of generic theories to be used together
with main HOL; the theory loader path already includes this directory
by default; the following existing theories have been moved here:
HOL/Induct/Multiset, HOL/Induct/Acc (as Accessible_Part), HOL/While
(as While_Combinator);
Goal "(case xs of [] => [] | y#ys => xs) = xs";
by(case_tac "xs" 1);