什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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Formal Sets
0 {  X- }% g  s0 @% Z& g; \' LA formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'.

) s2 T! t9 I. S$ G& P& p7 F0 XThe 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;
PFS:=PowerFormalSet(S);
PFS;
F := { 2, 4 };
2 F3 c. _% \3 L5 n' C( r/ GFF := { 2/3, 4 };

F in P;
FF in P;
2 \# I" F# ]- h: I) y3 V4 C. N F in PFS;
; H2 y8 L. x, U1 E% OFF in PFS;
Set of subsets of { 1 .. 5 }
  R9 I! D9 ~6 Q' CSet of formal subsets of { 1 .. 5 }
true
false
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& [- {2 z4 F; o) E1 H>>  F in PFS;
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Runtime error in 'in': Bad argument types


>> FF in PFS;
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# K& |! M0 F/ G. b2 b" X+ i4 S# ]9 ?Runtime error in 'in': Bad argument types