code: def build_tree_file_pathname(filename, directory_depth=8, pathname_separator_character=os.sep):
      
      return build_tree_pathname(filename, directory_depth, pathname_separator_character) + filename
docs: Return a file pathname which pathname is built of the specified
      number of sub-directories, and where each directory is named after the
      nth letter of the filename corresponding to the directory depth.
  
      Examples::
  
          >>> build_tree_file_pathname('foo.txt', 2, '/')
          'f/o/foo.txt'
          >>> build_tree_file_pathname('0123456789abcdef')
          '0/1/2/3/4/5/6/7/0123456789abcdef'
  
      @param filename: name of a file, with or without extension.
  
      @param directory_depth: number of sub-directories to be generated.
  
      @param pathname_separator_character: character to be used to separate
             pathname components, such as '/' for POSIX and '\\' for
             Windows.  If not defined, the default is the character used by
             the operating system ``os.sep``.
  
      @return: a file pathname.