Write current buffer into FILENAME, using a format based on FORMAT.
Constructs the actual format starting from FORMAT, then appending
any elements from the value of `buffer-file-format' with a non-nil
`preserve' flag (see the documentation of `format-alist'), if they
are not already present in FORMAT. It then updates `buffer-file-format'
with this format, making it the default for future saves.
If the buffer is already visiting a file, you can specify a
directory name as FILENAME, to write a file of the same old name
in that directory.
If optional third arg CONFIRM is non-nil, asks for confirmation before
overwriting an existing file. Interactively, requires confirmation
unless you supply a prefix argument.