Knownbits are unreliable for value tracking especially for carry operations.

  0011
  000?
+____
  0???

now we have 2^3 possible outcomes, however 0011 + 000? only could be 0100 or 0011, thus it can be represented as 2<=>3
We have to keep these values in a range. But range only works if we dont know the trail.

ex:

  0110
  00?0
+____
  ???0

now it could be 0110 or 1000, so 6 or 8.

We can do a mix of both ranges and bits to solve this.

??? - 0
info for ??? would be a range and 0 is knownbits