;; define a package that doesnt USE anything
CL-USER> (defpackage :foo)
#<PACKAGE "FOO">
;; go into that package
CL-USER> (in-package :foo)
#<COMMON-LISP:PACKAGE "FOO">
;; make a symbol (which will get interned into FOO)
FOO> 'in-package
IN-PACKAGE
;; try using the package CL, which already has "IN-PACKAGE" interned into it
FOO> (cl:use-package :cl)
ERROR: COMMON-LISP:USE-PACKAGE #<COMMON-LISP:PACKAGE "COMMON-LISP">
causes name-conflicts in #<COMMON-LISP:PACKAGE "FOO"> between
the following symbols:
FOO::IN-PACKAGE, COMMON-LISP:IN-PACKAGE