什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：

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'.
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2 R. d) O! t  H7 P& u7 Q$ ^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};
' B* m9 ]3 \7 g% a> P := PowerSet(S);
P;
PFS:=PowerFormalSet(S);
% D5 ~: _# j( T$ X; oPFS;
F := { 2, 4 };
FF := { 2/3, 4 };

F in P;
, T* ^3 n  S# i% z3 \2 @( @+ o2 O" mFF in P;
F in PFS;
" A* L% z1 `3 x& P2 u: Z- yFF in PFS;
: e" h0 |; u4 `) USet of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
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$ ?! g9 q+ ?+ ~- i! W0 }false
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: ]7 G% s2 d' N# p>>  F in PFS;
      ^
9 O( _& a7 w; jRuntime error in 'in': Bad argument types
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>> FF in PFS;
      ^
: k4 g2 b+ K3 t- ]$ J% eRuntime error in 'in': Bad argument types