Description: Define class difference, also called relative complement. Definition
5.12 of TakeutiZaring p. 20. For example,
( { 1 , 3 } \ { 1 , 8 } ) = { 3 } ( ex-dif ). Contrast this
operation with union ( A u. B ) ( df-un ) and intersection
( A i^i B ) ( df-in ). Several notations are used in the
literature; we chose the \ convention used in Definition 5.3 of
Eisenberg p. 67 instead of the more common minus sign to reserve the
latter for later use in, e.g., arithmetic. We will use the
terminology " A excludes B " to mean A \ B . We will
use " B is removed from A " to mean A \ { B } i.e. the
removal of an element or equivalently the exclusion of a singleton.
(Contributed by NM, 29-Apr-1994)