controls what happens when a CLtL1-style
form is processed. This refers to the specification in Common Lisp the Language, first Edition, which preceded ANSI Common Lisp and specified
as a function with keyword arguments.
Quietly use the CLtL1 definition of the
Signal a warning and use the old definition.
Signal a continuable error.
LispWorks User Guide and Reference Manual - 21 Dec 2011