Well, Poulis suggest to use BTD as Tum distacne. I was misled!!

Add two fuction about BTD: get_BTD and get_MaxBTD.

get_BTD(tran1, tran2)
Compute BTD of tran1 and tran2, using Bit vector.

get_MaxBTD
Compute BTD of a cluster. As Poulis define BTD of a cluster as the max of BTD(r1,r2) in cluster, where r1 and r2 are in cluster.