+@subsection How do I create a system definition where all the source files have a .cl extension?
+
+Starting with ASDF 2.014.14, you may just pass
+the builtin class @code{cl-source-file.cl} as
+the @code{:default-component-class} argument to @code{defsystem}:
+
+@lisp
+(defsystem my-cl-system
+ :default-component-class cl-source-file.cl
+ ...)
+@end lisp
+
+Another builtin class @code{cl-source-file.lsp} is offered
+for files ending in @file{.lsp}.
+
+If you want to use a different extension
+for which ASDF doesn't provide builtin support,
+or want to support versions of ASDF
+earlier than 2.014.14 (but later than 2.000),
+you can define a class as follows:
+
+@lisp
+;; Prologue: make sure we're using a sane package.
+(defpackage :my-asdf-extension
+ (:use :asdf :common-lisp)
+ (:export #:cl-source-file.lis))
+(in-package :my-asdf-extension)
+
+(defclass cl-source-file.lis (cl-source-file)
+ ((type :initform "lis")))
+@end lisp
+
+Then you can use it as follows:
+@lisp
+(defsystem my-cl-system
+ :default-component-class my-asdf-extension:cl-source-file.lis
+ ...)
+@end lisp
+
+Of course, if you're in the same package, e.g. in the same file,
+you won't need to use the package qualifier before @code{cl-source-file.lis}.
+Actually, if all you're doing is defining this class
+and using it in the same file without other fancy definitions,
+you might skip package complications:
+
+@lisp
+(in-package :asdf)
+(defclass cl-source-file.lis (cl-source-file)
+ ((type :initform "lis")))
+(defsystem my-cl-system
+ :default-component-class cl-source-file.lis
+ ...)
+@end lisp
+
+It is possible to achieve the same effect
+in a way that supports both ASDF 1 and ASDF 2,
+but really, friends don't let friends use ASDF 1.
+Please upgrade to ASDF 2.
+In short, though: do same as above, but
+@emph{before} you use the class in a @code{defsystem},
+you also define the following method:
+
+@lisp
+(defmethod source-file-type ((f cl-source-file.lis) (m module))
+ (declare (ignorable f m))
+ "lis")
+@end lisp
+