On 30.04.15 20:08, Ron Garret wrote: > (flet ((+ () t)) t) Not sure whether you consider this a solution, but did you try shadowing +? ? (defpackage shadow-+ (:use :cl)(:shadow "+")) #<Package "SHADOW-+"> ? (in-package :shadow-+) #<Package "SHADOW-+"> ? (flet ((+ () t)) t) T ?