+@item
+@vindex @sbpcl{+slot-unbound+}
+@findex @sbmop{standard-instance-access}
+@findex @sbmop{funcallable-standard-instance-access}
+distinguising unbound instance allocated slots from bound ones when
+using @code{standard-instance-access} and
+@code{funcallable-standard-instance-access} is possible by comparison
+to the constant @code{+slot-unbound+}.
+