什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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# t2 G7 u, r7 K$ FFormal Sets
A formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'.
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The only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing.
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S := { 1 .. 5};
> P := PowerSet(S);
P;
PFS:=PowerFormalSet(S);
PFS;
F := { 2, 4 };
7 b' a6 e) r5 g9 [- f& {% uFF := { 2/3, 4 };

+ L* F$ i5 y2 z7 R5 L; M F in P;
1 ^( q5 F& R6 P5 f7 {$ k* ?  j/ AFF in P;
/ [7 {; w- M+ d( W1 e  K F in PFS;
FF in PFS;
& s+ M6 i; k/ v$ a  aSet of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
true
0 f: u3 S$ w" Z- P; Q9 zfalse
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2 ~, r* ~8 s- ^! z9 m>>  F in PFS;
" x$ G5 p0 k0 j' ^) ?      ^
Runtime error in 'in': Bad argument types

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  T; o; D1 B: c: y1 n>> FF in PFS;
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$ y; o, |. G* z7 j( y, ^Runtime error in 'in': Bad argument types