什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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# x+ s; a2 O8 [! c6 f  _1 VFormal 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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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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* P3 P, s- Z; gS := { 1 .. 5};
/ i- i/ I/ w% Q* A8 N> P := PowerSet(S);
P;
PFS:=PowerFormalSet(S);
$ o$ V( x/ T  p+ [& o; g4 JPFS;
+ f4 h  v- \/ O6 PF := { 2, 4 };
FF := { 2/3, 4 };
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. F% [$ ~4 O. @2 z4 H# S3 t: O F in P;
FF in P;
- a! `; X9 ^4 ~ F in PFS;
' h9 ~0 F0 \8 |FF in PFS;
Set of subsets of { 1 .. 5 }
0 Z( ?" y  w: v6 H% ^Set of formal subsets of { 1 .. 5 }
. B& X8 g3 f( ?/ v7 {0 otrue
false

3 C3 |+ z' L3 @  L$ s! |, _1 m>>  F in PFS;
) f' J  o# o! E" O4 ^' N  t      ^
/ ^2 p3 u' o, l' Z( LRuntime error in 'in': Bad argument types
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: I( I( D2 W" h>> FF in PFS;
      ^
Runtime error in 'in': Bad argument types