什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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. s. V8 G" |  M, G9 b9 JFormal Sets
A formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'.

5 x# \# Y# s4 |3 |! U; rThe 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;
  j4 h  C- M" B- {PFS:=PowerFormalSet(S);
( n% c! p. o% q7 T* C4 |; EPFS;
3 U2 D/ h5 q) ZF := { 2, 4 };
FF := { 2/3, 4 };

F in P;
FF in P;
F in PFS;
FF in PFS;
Set of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
true
5 ^! \' t0 F3 @$ p# t% R' Z! T7 Jfalse

>>  F in PFS;
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2 b- N: D$ [$ y. {; ]4 a2 cRuntime error in 'in': Bad argument types
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$ J1 X. Y1 ^# V0 v4 f3 T>> FF in PFS;
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Runtime error in 'in': Bad argument types