added addD2, addE2, addSD2, and addSE2
improved addbefore and addSbefore
improved mechanism for unsafe wrappers
consts valif :: ifex => (nat => bool) => bool
primrec
"valif (CIF b) env = b"
"valif (VIF x) env = env x"
"valif (IF b t e) env = (if valif b env then valif t env
else valif e env)"