什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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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'.

6 I/ [8 d! ^7 \8 Y6 }3 WThe 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;
! `# I/ M4 D" V4 }2 cPFS:=PowerFormalSet(S);
; W0 g- z5 a5 h" w4 y4 jPFS;
F := { 2, 4 };
7 J/ N" u) h# L8 P8 A$ ^7 DFF := { 2/3, 4 };
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* Y" b* v; f2 l$ b" {: ]# R# B F in P;
FF in P;
) U5 m3 g: t' _! P. u F in PFS;
0 v, y% ~# G6 M, t1 J1 OFF in PFS;
8 l; F! S/ P# d. U6 W8 ESet of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
true
false

>>  F in PFS;
      ^
Runtime error in 'in': Bad argument types
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( i; c( K# K% @>> FF in PFS;
      ^
Runtime error in 'in': Bad argument types