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'.

& F# k; @) r/ y6 C5 TThe only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing.
& _1 a; m/ r1 P& s: J1 U1 `2 ]  s
( F7 i& X# s0 j) z
4 _+ p: a! E5 R: B5 \! i; x2 p
S := { 1 .. 5};
+ l! n. O0 u) g! d7 c& @; F> P := PowerSet(S);
) m9 V  ~! K7 pP;
PFS:=PowerFormalSet(S);
PFS;
5 ?. f9 R* Y; yF := { 2, 4 };
FF := { 2/3, 4 };
$ {- H. K5 N) U9 Z. J; D' |) Q
% D5 U+ j) L6 X0 L5 U) j F in P;
5 v' Q+ e: {& s# LFF in P;
F in PFS;
! `# T  K6 v$ {4 V5 X7 BFF in PFS;
" q  j8 u# e4 w0 n) X, USet of subsets of { 1 .. 5 }
- j+ f/ E$ A" _# [* D% fSet of formal subsets of { 1 .. 5 }
; f$ `) e0 \% X# I8 Atrue
+ @, Y' f& f: V/ q; ?" E% ~) cfalse
1 G* y0 Z9 [' ]
% |0 m' I! E: B$ b8 Q0 N$ {>>  F in PFS;
( o3 h- @( ~3 E9 \  S; }, {      ^
. ]% r% W6 w# E. V% g9 k  LRuntime error in 'in': Bad argument types


>> FF in PFS;
* i4 k% i5 `) c9 L      ^
Runtime error in 'in': Bad argument types