- Moved abs_def to drule.ML
- elim_defs now takes a boolean argument which controls the automatic
expansion of theorems mentioning constants whose definitions are
eliminated
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)"