Generate and print a PostScript syntactic chart image of the buffer.
When called with a numeric prefix argument (C-u), prompts the user for
the name of a file to save the PostScript image in, instead of sending
it to the printer.
More specifically, the FILENAME argument is treated as follows: if it
is nil, send the image to the printer. If FILENAME is a string, save
the PostScript image in a file with that name. If FILENAME is a
number, prompt the user for the name of the file to save in. (fn &optional FILENAME)