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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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};
- y- H$ \8 z8 b3 b) u/ I> P := PowerSet(S);
P;
& r! r6 ^4 Y5 ]PFS:=PowerFormalSet(S);
PFS;
- C, B3 o* f; @7 m* x: IF := { 2, 4 };
FF := { 2/3, 4 };

4 Z) R7 @: ~$ g/ m F in P;
3 F  n, a1 O& o9 yFF in P;
F in PFS;
) Z. a& x& b% y8 i/ MFF in PFS;
/ T3 r1 O  y7 P: xSet of subsets of { 1 .. 5 }
Set of formal subsets of { 1 .. 5 }
& }" ~; U; J7 J- a+ a. n& C  ptrue
3 E5 n, q! q. g  s* P" ]false
( l$ g3 \5 {! W- R
>>  F in PFS;
      ^
Runtime error in 'in': Bad argument types


! R1 v9 H" c' u5 z>> FF in PFS;
      ^
, O0 m# l" ^0 \7 WRuntime error in 'in': Bad argument types