a $mu$-way $(v,k,t)$ $trade$ of volume $m$ consists of $mu$ disjoint collections $t_1$, $t_2, dots t_{mu}$, each of $m$ blocks, such that for every $t$-subset of $v$-set $v$ the number of blocks containing this t-subset is the same in each $t_i (1leq i leq mu)$. in other words any pair of collections ${t_i,t_j}$, $1leq i< j leq mu$ is a $(v,k,t)$ trade of volume $m$. in th...