input: Write a function to convert an HTML string to plain text without using any libraries. If the HTML string is empty, return an empty string.
  
  assert html_to_text("<html><body>Hello World!</body></html>")=="Hello World!"
  assert html_to_text("<p>This is a paragraph.</p>")=="This is a paragraph."
  assert html_to_text("<a href='link'>Click here</a>")=="Click here"
  assert html_to_text("<h1>Header</h1>")=="Header"
  assert html_to_text("")==""
output: def html_to_text(html):
      if not html:
          return ""
      return ''.join(filter(lambda x: x not in ('<', '>'), html)).replace('/', '').replace('=', '').replace("'", '').replace('"', '')