code: def normpath(path):
      # type: (Text) -> Text
      
      if path in "/":
          return path
  
      # An early out if there is no need to normalize this path
      if not _requires_normalization(path):
          return path.rstrip("/")
  
      prefix = "/" if path.startswith("/") else ""
      components = []  # type: List[Text]
      try:
          for component in path.split("/"):
              if component in "..":  # True for '..', '.', and ''
                  if component == "..":
                      components.pop()
              else:
                  components.append(component)
      except IndexError:
          raise IllegalBackReference(path)
      return prefix + "/".join(components)
docs: Normalize a path.
  
      This function simplifies a path by collapsing back-references
      and removing duplicated separators.
  
      Arguments:
          path (str): Path to normalize.
  
      Returns:
          str: A valid FS path.
  
      Example:
          >>> normpath("/foo//bar/frob/../baz")
          '/foo/bar/baz'
          >>> normpath("foo/../../bar")
          Traceback (most recent call last)
              ...
          IllegalBackReference: path 'foo/../../bar' contains back-references outside of filesystem"