The sentence {tt (item $tau_1$ $tau_2$)} is true if and only if the object denoted by $tau_2$ is a non-empty list and the object denoted by $tau_1$ is either the first item of that list or an item in the rest of the list.
(=> (Item ?X ?List)
(And (Not (Null ?List))
(Or (= ?X (First ?List)) (Item ?X (Rest ?List)))))