canonical 'cases'/'induct' rules for n-tuples (n=3..7)
(really belongs to theory Product_Type, but doesn't work there yet)
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)"