if-does-not-exist

[Daniel L. Weinreb <DLW@SCRC-QUABBIN.ARPA>]

The description of the default value of the :if-does-not-exist argument
to open, on page 422, confused our implementor.  Strictly read, the
manual is ambiguous about the default value, in the case where
:direction is :probe and :if-exists is :overwrite or :append.  The
intention of the manual is probably that you realize that :if-exists is
not defined for :probe, but this is not completely clear from the text.
This should be clarified in the next edition.