The sentence {tt (proper-subset $tau_1$ $tau_2$)} is true if the set denoted by $tau_1$ is a subset of the set denoted by $tau_2$ but not vice-versa.
(<=> (Proper-Subset ?S1 ?S2) (And (Subset ?S1 ?S2) (Not (Subset ?S2 ?S1))))
(<=> (Proper-Subset ?S1 ?S2) (And (Subset ?S1 ?S2) (Not (Subset ?S2 ?S1)))) (Exists (?U) (And (Bounded ?U) (Not (Empty ?U)) (Forall (?X) (=> (Member ?X ?U) (Exists (?Y) (And (Member ?Y ?U) (Proper-Subset ?X ?Y)))))))