Question

I define a package

 [10]> (defpackage :abc)
 #<PACKAGE ABC>
 [11]>

I use that package

 [15]> (in-package :abc)
 #<PACKAGE ABC>
 ABC[16]>

How to exit ABC[16] to [17] ?

Thank you~

Answer 1

Strictly speaking you don’t use the package with in-package but you
set the current package. To use a package you’ve to use use-package.

Back to your problem, you’ve defined a new package (without using :cl
so you’ll have to use (cl:+ 1 2) to do an addition) and set it as
the current package. COMMON-LISP-USER a.k.a. CL-USER is the
current package when you start your lisp system, thus to have it back
you just have to do (cl:in-package :cl-user).

Here is a nice tutorial on packages in Common Lisp. This chapter
in PCL is also very good. But the reference is still the CLHS