什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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" ^3 P9 [: m6 X* w3 o  CFormal 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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4 p5 p$ D1 j. s" G8 d% PThe only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing.



S := { 1 .. 5};
  \7 L: U+ M6 A+ P. K! l( c$ A> P := PowerSet(S);
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' k* R& p. y3 B" {PFS:=PowerFormalSet(S);
, I. O+ t, W& ZPFS;
F := { 2, 4 };
, M) W; y6 i7 ?FF := { 2/3, 4 };
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F in P;
8 O' x  M& M: v$ e, hFF in P;
F in PFS;
0 G2 u( Z  n. o! m8 O, A* o) }FF in PFS;
Set of subsets of { 1 .. 5 }
0 s% P9 t" Y  C1 lSet of formal subsets of { 1 .. 5 }
true
false

( T2 E/ ?1 @; I: T- o>>  F in PFS;
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Runtime error in 'in': Bad argument types
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>> FF in PFS;
      ^
+ G7 X1 g. ]0 Q) E" \7 x( G7 WRuntime error in 'in': Bad argument types