<> (`?: ?0 ?1) ((Int ?:))
<> (fib ?n) (?n (Int 0) (Int 1) Fib)
<> ((Int 0) (Int ?0) (Int ?1) Fib) ?0
<> ((Int ?x) (Int ?y) (Int ?z) Fib) (`- ?x 1 (Int ?z) `+ ?z ?y Fib)
fib (Int 11)