major code change: refute can now handle any Isabelle term, adds certain axioms automatically, and can handle inductive datatypes (but not yet recursion over them)
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))"