canonical 'cases'/'induct' rules for n-tuples (n=3..7)
(really belongs to theory Product_Type, but doesn't work there yet)
consts norm :: ifex => ifex
primrec
"norm (CIF b) = CIF b"
"norm (VIF x) = VIF x"
"norm (IF b t e) = normif b (norm t) (norm e)"