The sentence {tt (subset $tau_1$ $tau_2$)} is true if and only if $tau_1$ and $tau_2$ are sets and the objects in the set denoted by $tau_1$ are contained in the set denoted by $tau_2$.
(<=> (Subset ?S1 ?S2)
(And (Set ?S1)
(Set ?S2)
(Forall (?X) (=> (Member ?X ?S1) (Member ?X ?S2)))))
(=> (Subset $X $Y) (Set $Y))
(=> (Subset $X $Y) (Set $X))
(<=> (Subset ?S1 ?S2)
(And (Set ?S1)
(Set ?S2)
(Forall (?X) (=> (Member ?X ?S1) (Member ?X ?S2)))))
(Subrelation-Of Proper-Subset Subset)
(<=> (Proper-Subset ?S1 ?S2)
(And (Subset ?S1 ?S2) (Not (Subset ?S2 ?S1))))
(<=> (Set-Cover ?S @Sets) (Subset ?S (Union @Sets)))
(=> (Bounded ?V) (Bounded (Setofall ?U (Subset ?U ?V))))