defsystem alone is not enough, should be asdf:defsystem

or, as done sometimes, (in-package :asdf-user)

otherwise newcomers cannot LOAD .asd files :S

with thanks to Marc R.

getting-started.md

@@ -403,7 +403,7 @@ it will create a directory structure like this:
 Where `my-project.asd` resembles this:
 
 ~~~lisp
-(defsystem "my-project"
+(asdf:defsystem "my-project"
   :version "0.1.0"
   :author ""
   :license ""

systems.md

@@ -77,7 +77,7 @@ you create a system definition file called `foobar.asd`,
 with the following contents:
 
 ~~~lisp
-(defsystem "foobar"
+(asdf:defsystem "foobar"
   :depends-on ("alexandria" "trivia")
   :components ((:file "foobar")))
 ~~~
@@ -143,12 +143,12 @@ The simplest way to write tests is to have a file `foobar-tests.lisp`
 and modify the above `foobar.asd` as follows:
 
 ~~~lisp
-(defsystem "foobar"
+(asdf:defsystem "foobar"
     :depends-on ("alexandria" "trivia")
     :components ((:file "foobar"))
     :in-order-to ((test-op (test-op "foobar/tests"))))
 
-(defsystem "foobar/tests"
+(asdf:defsystem "foobar/tests"
     :depends-on ("foobar" "fiveam")
     :components ((:file "foobar-tests"))
     :perform (test-op (o c) (symbol-call :fiveam '#:run! :foobar)))

testing.md

@@ -358,11 +358,11 @@ See also [cl-quickcheck](https://github.com/mcandre/cl-quickcheck) and [Check-it
 So it would be nice to provide a one-line trigger to test our `my-system` system. Recall that we said it is better to provide a root suite? Here is the reason:
 
 ~~~lisp
-(defsystem my-system
+(asdf:defsystem my-system
   ;; Parts omitted.
   :in-order-to ((test-op (test-op :my-system/test))))
 
-(defsystem mitogrator/test
+(asdf:defsystem mitogrator/test
   ;; Parts omitted.
   :perform (test-op (op c)
                     (symbol-call :fiveam :run!