什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：
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3 D# b8 z5 K! I6 g& ]- BFormal Sets
A formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'.

+ M; O& r0 b; D" t! zThe 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 o! f; S3 J6 lS := { 1 .. 5};
> P := PowerSet(S);
: Q* @# b+ i7 ^% O  ?P;
PFS:=PowerFormalSet(S);
PFS;
6 u9 B+ r% Y% @/ SF := { 2, 4 };
4 @! G) M/ }4 \FF := { 2/3, 4 };

F in P;
FF in P;
F in PFS;
* j* ~* @7 q0 s5 z' CFF in PFS;
% G9 c  X8 g0 i) t8 u  Y0 i1 z4 }Set of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
" h1 [) P. k6 z6 [* Q* {- Vtrue
false
  _9 h0 ]; b0 \. \: l
>>  F in PFS;
      ^
; z4 q' o5 l) w. {9 C9 MRuntime error in 'in': Bad argument types
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1 N1 B- n. u) O6 t) N3 Z4 v# S: G, e>> FF in PFS;
      ^
Runtime error in 'in': Bad argument types