Write output from interpreter since last input to FILENAME.
Any prompt at the end of the output is not written.
If the optional argument APPEND (the prefix argument when interactive)
is non-nil, the output is appended to the file instead.
If the optional argument MUSTBENEW is non-nil, check for an existing
file with the same name. If MUSTBENEW is `excl', that means to get an
error if the file already exists; never overwrite. If MUSTBENEW is
neither nil nor `excl', that means ask for confirmation before
overwriting, but do go ahead and overwrite the file if the user
confirms. When interactive, MUSTBENEW is nil when appending, and t
otherwise. (fn FILENAME &optional APPEND MUSTBENEW)