哪位能给理解下:. Z2 X, q$ {# i* Y0 N( b" V
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Formal Sets7 i# \& }! J6 Y# _
A formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'. # c1 ^8 A q: w) J% A9 W* h G2 }3 H) ]* G
The only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing. $ s. B* ]; k! e4 r! w( z
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FF in P; # Z, }3 X; ?8 b v- e F in PFS;" y# {; ? [; E
FF in PFS;* V) H) o. U6 j' P! J1 Y* s! Q
Set of subsets of { 1 .. 5 }8 s$ f/ T; q' q( B. i# U
Set of formal subsets of { 1 .. 5 }7 o. s0 Z0 @! b- l; X8 k+ z$ y
true . J, D2 @ Z, ]. }3 ^false ; v5 a% J( r& w. e; a q) F( B2 R! R4 z% m2 W: G6 V
>> F in PFS;% Z& q0 J2 K- l- {. z
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Runtime error in 'in': Bad argument types/ C* ~: a; J6 m6 m4 d3 Z+ ^4 H
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6 h2 f8 K! y4 x8 a$ W>> FF in PFS; ; ~. |' F) ]# c9 g0 L6 n ^ 7 h- h2 P8 w, Z% n. sRuntime error in 'in': Bad argument types