Process an option of the form (:OPTION . l), where l must be a SUBSETP of a given set.
This function checks for an option that is specified as (:OPTION . l), where l must be a proper list and a SUBSETP of the-set. Thus (:OPTION) specifies the empty set, and :OPTION by itself is illegal. If the option is missing then the default value is returned, otherwise the subset l is returned.