什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：

0 U1 ~2 J/ V5 a0 g/ P2 ~/ r/ TFormal 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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& O! D1 _  ~/ c' z3 q1 y" eS := { 1 .. 5};
' x: a# ~# r* I1 d/ e- W# F0 v) `. T5 l1 Q> P := PowerSet(S);
P;
PFS:=PowerFormalSet(S);
: N! K! c# T2 M9 f( L/ k; q1 TPFS;
F := { 2, 4 };
5 ?! Y& y& E/ @% nFF := { 2/3, 4 };

( G' Y5 Y( D$ D F in P;
FF in P;
& F! _/ b/ X5 l( B) W6 L' k F in PFS;
5 o) p$ S+ G( E% w: T! D; rFF in PFS;
) ?( T3 m! J3 ^9 K2 N& I$ USet of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
) k* T0 B# @3 x- A. |) ltrue
false

; B" H: z6 k9 I0 Q>>  F in PFS;
      ^
. c0 k' o3 b  D- y; |- [Runtime error in 'in': Bad argument types
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* R& `, y) [8 Z# G; N! t$ G3 ]( P
( W- f- ?4 J6 u>> FF in PFS;
      ^
Runtime error in 'in': Bad argument types