input: 
  def html_to_text(html_string: str) -> str:
      """ Convert HTML to plain text without using any libraries.
      
      >>> html_to_text('<html><body>Hello, <b>world</b>!</body></html>')
      'Hello, world!'
      
      >>> html_to_text('<div>This is a <span>test</span> string.</div>')
      'This is a test string.'
      """
      output = ''
      in_tag = False
      for char in html_string:
          if char == '<':
              in_tag = True
          elif char == '>':
              in_tag = False
          elif in_tag == False:
              output += char
      return output
  
output:     output = ''
      in_tag = False
      for char in html_string:
          if char == '<':
              in_tag = True
          elif char == '>':
              in_tag = False
          elif in_tag == False:
              output += char
      return output