说明：  A major obstacle that stands in the way of efficient test response compaction are the unknown values (x-values) captured by scan cells during testing. If test responses with x-values are compacted, some of the outputs of the compactor may also take unknown values and the correctness of the compactor inputs cannot be verified at the compactor outputs. The presence of x-values hence reduces observability of (nonx) scan cells that may lead to a reduction of test quality and/or limited compaction rates