docs: final bbBlock findBlockHelper(int pos, int lo, int hi)
      {
          bbBlock block;
          int     ii;
          if ((hi - lo) <= 3) {
              for (ii=lo; ii<hi; ii++) {
                  block = this._blocks.get(ii);
                  if (pos > block._offset + block._limit) continue;
                  if (block.containsForRead(pos)) {
                      return block;
                  }
                  if (block._offset >= pos) break;
              }
              return this._blocks.get(ii - 1);    // this will always be > 0
          }
          int mid = (hi + lo) / 2;
          block = this._blocks.get(mid);
          assert block != null;
          if (block._offset > pos) {
              return findBlockHelper(pos, lo, mid);
          }
          return findBlockHelper(pos, mid, hi);
      }