code: def markup_line(text, offset, marker='>>!<<'):
      
  
      begin = text.rfind('\n', 0, offset)
      begin += 1
  
      end = text.find('\n', offset)
  
      if end == -1:
          end = len(text)
  
      return text[begin:offset] + marker + text[offset:end]
docs: Insert `marker` at `offset` into `text`, and return the marked
      line.
  
      .. code-block:: python
  
         >>> markup_line('0\\n1234\\n56', 3)
         1>>!<<234