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A pair of disjoint collections of $k$-subsetes of a $v$-set
$X$, $\{T_1,T_2\}$, is called a $t$-$(v,k)$ trade if every $t$-subset of
$X$ appears precisely the same number of times in $T_1$ as in $T_2$.
The purpose of this paper is to discuss the structures and properties of
these combinatorial objects, review some of their applications, and present
an update on the subject.
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