code: def user_avatar_url(username, size=64, default="retro"):
      
      openid = "http://{}.id.fedoraproject.org/".format(username)
      return libravatar_url(openid=openid, size=size, default=default)
docs: Get the avatar URL of the provided Fedora username.
  
      The URL is returned from the Libravatar service.
  
      Args:
          username (str): The username to get the avatar of.
          size (int): Size of the avatar in pixels (it's a square).
          default (str): Default avatar to return if not found.
      Returns:
          str: The URL to the avatar image.