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 value :: boolex => (nat => bool) => bool
primrec
"value (Const b) env = b"
"value (Var x) env = env x"
"value (Neg b) env = (~ value b env)"
"value (And b c) env = (value b env & value c env)"