maintains a sorted list of variables having non-zero unary costs in order to make NC propagation incremental.
- variables are sorted into buckets
- each bucket is associated to a single interval of non-zero costs (using a power-of-two scaling, first bucket interval is [1,2[, second interval is [2,4[, etc.)
- each variable is inserted into the bucket corresponding to its largest unary cost in its domain
- variables having all unary costs equal to zero do not belong to any bucket
NC propagation will revise only variables in the buckets associated to costs sufficiently large wrt current objective bounds.