val option_rws = Let_def :: Option.option.simps;
val SS = arith_ss addsimps option_rws;
val [prem] = goal Option.thy "P(opt) ==> P(None) | (? x. P(Some(x)))";
br (prem RS rev_mp) 1;
by (Option.option.induct_tac "opt" 1);
by (ALLGOALS(fast_tac HOL_cs));
val optE = standard(result() RS disjE);