In cases where no rule applies Shen will raise an error and offer to track the offending function.
(0-) (define likes
tom dick -> yes
dick harry -> yes
harry tom -> yes)
(fn likes)
(1-) (likes tom dick)
yes
(2-) (likes dick fred)
partial function likes
track likes? (y/n) n
(track f) will trace a function f and (untrack f) will untrace it.