+ ;; FIXME: The type of PEEK-TYPE is also declared in a DEFKNOWN, but
+ ;; the compiler doesn't seem to be smart enough to go from there to
+ ;; imposing a type check. Figure out why (because PEEK-TYPE is an
+ ;; &OPTIONAL argument?) and fix it, and then this explicit type
+ ;; check can go away.
+ (unless (typep peek-type '(or character boolean))
+ (error 'simple-type-error
+ :datum peek-type
+ :expected-type '(or character boolean)
+ :format-control "~@<bad PEEK-TYPE=~S, ~_expected ~S~:>"
+ :format-arguments (list peek-type '(or character boolean))))