Functions on this list take four arguments--the pathname of the file; an external format spec; a vector of element-type
which contains the first bytes of the file; and a non-negative integer which is the maximum extent of buffer to be searched. This length argument is 0 in the case that the file does not exist, or the direction is
. They return an external format spec, which normally is either
unmodified, or the result of merging
with another external format spec via
If you want open and so on, when opening a file for input, to inspect the attribute line and then fall back to a default if no attribute line is found, then set the variable to this value:
There are further examples in Guessing the external format.
LispWorks User Guide and Reference Manual - 21 Dec 2011