什麽是形式集合Formal Sets/PowerFormalSet？
哪位能给理解下：Formal Sets
A formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'.
The only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing.
S := { 1 .. 5};
> P := PowerSet(S);
P;
PFS:=PowerFormalSet(S);
PFS;
F := { 2, 4 };
FF := { 2/3, 4 };
F in P;
FF in P;
F in PFS;
FF in PFS;
Set of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
true
false
>> F in PFS;
^
Runtime error in 'in': Bad argument types
>> FF in PFS;
^
Runtime error in 'in': Bad argument types
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