* X" D; ~5 U3 Z- _3 M1 D- ^Formal Sets' w* s( i& `; ?; _
A formal set consists of the subset of elements of some carrier set (structure) on which a certain predicate assumes the value `true'. ) O1 C' c+ i4 N/ h) G " e! e0 j% l+ h6 y$ T' G8 fThe only set-theoretic operations that can be performed on formal sets are union, intersection, difference and symmetric difference, and element membership testing. 8 b+ B* z& P$ }% I- `- v7 G# F' L: M/ x9 t7 d