什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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Formal Sets
8 D2 W9 f( s' G( }9 J  V4 YA 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.
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S := { 1 .. 5};
> P := PowerSet(S);
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PFS:=PowerFormalSet(S);
PFS;
% R; e# g! P9 eF := { 2, 4 };
FF := { 2/3, 4 };
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F in P;
1 k* H3 o: S, y( iFF in P;
F in PFS;
FF in PFS;
Set of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
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5 H" x# [( ]" nfalse

>>  F in PFS;
1 k" I2 {* _/ x3 C  Z) M      ^
Runtime error in 'in': Bad argument types
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>> FF in PFS;
      ^
' s/ e0 {+ M; F0 s* M. Y5 ]8 q8 dRuntime error in 'in': Bad argument types