simplified proofs for splitI and splitD, added splitD'
added split_conv_tac (also to claset()) as an optimization
made split_all_tac safe introducing safe_full_simp_tac,EXISTING PROOFS MAY FAIL
consts normal :: ifex => bool
primrec
"normal(CIF b) = True"
"normal(VIF x) = True"
"normal(IF b t e) = (normal t & normal e &
(case b of CIF b => True | VIF x => True | IF x y z => False))"