public override StringRebuilder GetLeaf(int position, out int offset)
 {
     if (position < _left.Length)
     {
         return(_left.GetLeaf(position, out offset));
     }
     else
     {
         var leaf = _right.GetLeaf(position - _left.Length, out offset);
         offset += _left.Length;
         return(leaf);
     }
 }