def insert_block(position, anchor_block, new_block)
      case position
      when :before
        anchor_block.xml.previous_sibling = new_block.xml
      when :after
        anchor_block.xml.next_sibling = new_block.xml
      else
        raise "position argument must be one of :before, :after"
      end
    end
def insert_block(position, anchor_block, new_block)
      case position
      when :before
        anchor_block.xml.previous_sibling = new_block.xml
      when :after
        anchor_block.xml.next_sibling = new_block.xml
      else
        raise "position argument must be one of :before, :after"
      end
    end
insert the xml of a content block :before or :after the anchor_block