什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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Formal Sets
3 N7 ~, z! O8 j. ?6 u- |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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; D0 Q( a9 }# ^& K2 tThe 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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! @5 L* c8 }7 m# eS := { 1 .. 5};
/ ~  R4 H; {4 l" j6 Y, T) z> P := PowerSet(S);
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# H$ P# }3 O4 y7 q/ \: c! }PFS:=PowerFormalSet(S);
( x: D1 w' O7 S+ [2 X0 }- CPFS;
F := { 2, 4 };
* L4 C' P5 S( x4 U1 q- _FF := { 2/3, 4 };
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F in P;
* R* Y7 {  _9 |/ z1 b: I- uFF in P;
6 @* Q1 N+ q& W3 C! N5 b F in PFS;
FF in PFS;
: _5 O" t. N9 X. w% ~Set of subsets of { 1 .. 5 }
/ S, o% d1 m: ?9 o1 s* T9 ^Set of formal subsets of { 1 .. 5 }
true
  Q' M( }; a4 @, }$ V) @1 B9 Ffalse
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3 |) V$ K2 ^7 U2 x! z( `# l>>  F in PFS;
      ^
3 q& `0 B: P+ m, A9 n& }5 K& LRuntime error in 'in': Bad argument types


" g2 r2 C2 r, i; I8 U% {8 m>> FF in PFS;
7 n+ b3 I4 Y' h  W, c( c# o- W      ^
Runtime error in 'in': Bad argument types