code: @Override
      public Operation zCombinedAnd(Operation op)
      {
          if (op instanceof IsNullOperation)
          {
              return zCombineWithNullOperation((IsNullOperation) op);
          }
          return op.zCombinedAnd(this);
      }
docs: /*
  returns the combined and operation. Many operations are more efficient when combined.
  This method is internal to Mithra's operation processing.