code: public Bits diffSet(Bits xs) {
          Assert.check(currentState != BitsState.UNKNOWN);
          for (int i = 0; i < bits.length; i++) {
              if (i < xs.bits.length) {
                  bits[i] = bits[i] & ~xs.bits[i];
              }
          }
          currentState = BitsState.NORMAL;
          return this;
      }
docs: this set = this set \ xs.