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)
consts fib :: nat => nat
recdef fib "measure(%n. n)"
"fib 0 = 0"
"fib 1 = 1"
"fib (Suc(Suc x)) = fib x + fib (Suc x)"