什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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, S" s/ o0 z$ }7 VFormal Sets
* }1 E8 g3 e' N0 E+ c6 rA formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'.

9 L- I8 O) t. \# nThe only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing.
) b$ d7 V4 Y$ K: K
$ }; R2 j7 {2 P$ p
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/ G" v, h& K% Q6 h6 zS := { 1 .. 5};
- L  y4 I' Z) B* S2 B5 e> P := PowerSet(S);
P;
PFS:=PowerFormalSet(S);
PFS;
% o. ]9 \. g% y* A4 P$ UF := { 2, 4 };
* H8 r9 Q; }) B* T) HFF := { 2/3, 4 };

F in P;
% D9 r$ c* S* |9 CFF in P;
F in PFS;
FF in PFS;
1 t/ u! }  Q/ {+ L  x* ?; `$ DSet of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
% L; y$ k* s2 m3 }true
false
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>>  F in PFS;
      ^
Runtime error in 'in': Bad argument types
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# }- K% j2 w% s- I! o>> FF in PFS;
( |+ u6 H& n/ K) h4 N      ^
/ I3 ]6 w2 N- d: e8 d, S+ W1 bRuntime error in 'in': Bad argument types