什麽是形式集合Formal Sets/PowerFormalSet？

lilianjie

哪位能给理解下：

Formal Sets
/ z0 n9 ~* ^& L0 BA formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'.
. @4 a* U, g6 F& G4 t" k' Z
5 G* F9 n1 d! D6 r/ D3 ?, P2 i0 d* TThe only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing.

! N' R1 l3 D6 @- l0 r

S := { 1 .. 5};
> P := PowerSet(S);
P;
6 u# H: \3 w( z0 }  _PFS:=PowerFormalSet(S);
PFS;
F := { 2, 4 };
7 \( H$ C9 M8 `% U) N- Y4 j8 mFF := { 2/3, 4 };

F in P;
FF in P;
F in PFS;
) P* b% y" T# k! ~8 b% bFF in PFS;
Set of subsets of { 1 .. 5 }
9 u3 E! I2 k, B# N5 YSet of formal subsets of { 1 .. 5 }
% `. Z3 Q4 g% U* Wtrue
false
; y* l& S1 w" Z2 _& @
' J: ]+ P* `5 \7 ~! b: t>>  F in PFS;
      ^
/ s0 S- g0 Y+ X) _* S  {Runtime error in 'in': Bad argument types

6 E1 u6 [3 S( O# K2 {% N
>> FF in PFS;
      ^
Runtime error in 'in': Bad argument types