canonical 'cases'/'induct' rules for n-tuples (n=3..7)
(really belongs to theory Product_Type, but doesn't work there yet)
GCD = WF_Rel +
consts gcd :: "nat*nat => nat"
recdef gcd "measure ((%(m,n).n))"
simpset "simpset() addsimps [mod_less_divisor]"
"gcd (m, n) = (if n=0 then m else gcd(n, m mod n))"
end