added addD2, addE2, addSD2, and addSE2
improved addbefore and addSbefore
improved mechanism for unsafe wrappers
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)"