code: Object insertByLeftShift(
      int               ix,
      Object            new1)
    {
      Object old1 = leftMostKey();
      leftShift(ix);
      _nodeKey[ix] = new1;
      if (midPoint() > rightMostIndex())
        _nodeKey[midPoint()] = rightMostKey();
      return old1;
    }
docs: Insert a new key by overlaying the left-most key, shifting
  other keys left and inserting the new key at the appropriate
  insert point.
  
  @param ix The insert point within the node.
  @param new1 The new key to be inserted.
  @return the key that used to be the left-most key in the node.