docstr: Add a `SubstituteTagInlineProcessor` to Markdown. 
docstr_tok: ["Add","a","`","SubstituteTagInlineProcessor","`","to","Markdown","."]
code: def extendMarkdown(self, md):
      """Add a `SubstituteTagInlineProcessor` to Markdown."""
      br_tag = SubstituteTagInlineProcessor(BR_RE, "br")
      md.inlinePatterns.register(br_tag, "nl", 5)
  
imports: from __future__ import annotations
  from .None import Extension
  from ..inlinepatterns import SubstituteTagInlineProcessor
label_desc: Use image node_id 1 for calling the Nl2BrExtension obj's underlying member method code with example usage: obj.extendMarkdown(md) without return types