class_docstr: Use this mixin to make Demo classes with less visual clutter.
  
  Demos using this mixin will clear the screen before every block and use
  blank marquees.
  
  Note that in order for the methods defined here to actually override those
  of the classes it's mixed with, it must go /first/ in the inheritance
  tree.  For example:
  
      class ClearIPDemo(ClearMixin,IPythonDemo): pass
  
  will provide an IPythonDemo class with the mixin's features.
docstr: Blank marquee that returns '' no matter what the input.
docstr_tok: ["Blank","marquee","that","returns","``","no","matter","what","the","input","."]
code: def marquee(self, txt="", width=78, mark="*"):
      """Blank marquee that returns '' no matter what the input."""
      return ""
  
imports: import os
  import re
  import shlex
  import sys
  import pygments
  from pathlib import Path
  from IPython.utils.text import marquee
  from IPython.utils import openpy
  from IPython.utils import py3compat
label_desc: Use image node_id 1 for calling the ClearMixin obj's underlying member method code with example usage: obj.marquee(txt, width, mark) and returns: str