Return a stream which reads from or writes to filename. Defined keywords:

:direction

One of :input, :output, :io, or :probe.

:element-type

Type of object to read or write, default base-char.

:external-format

The external format to use. The default value is :default for which the value of ext:*default-external-format* is used. The external format may be a keyword representing a character set to use for character streams. For local file streams, the external format may also be a list of formats that convert from the desired character or binary stream to the binary stream read and/or written to the file. Leading character based formats are ignored for binary streams.

:if-exists

One of :error, :new-version, :rename, :rename-and-delete, :overwrite, :append, :supersede or nil.

:if-does-not-exist

One of :error, :create or nil.

See the ANSI CL manual for details.