gnu: Add emacs-sly.

* gnu/packages/emacs.scm (emacs-sly): New variable.

gnu/packages/emacs.scm

@@ -4286,6 +4286,52 @@ are pretty much the same (and SLIME served as the principle inspiration for
 CIDER).")
     (license license:gpl3+)))
 
+;; There hasn't been a tag or release since 2015, so we take the latest
+;; commit.
+(define-public emacs-sly
+  (let ((commit "486bfbe95612bcdc0960c490207970a188e0fbb9")
+        (revision "1"))
+    (package
+      (name "emacs-sly")
+      (version (string-append "1.0.0-" revision "." (string-take commit 9)))
+      (source
+       (origin
+         (method git-fetch)
+         (uri (git-reference
+               (url "https://github.com/joaotavora/sly.git")
+               (commit commit)))
+         (sha256
+          (base32
+           "0ib4q4k3h3qn88pymyjjmlmnpizdn1mfg5gpk5a715nqsgxlg09l"))))
+      (build-system emacs-build-system)
+      (arguments
+       `(#:include (cons "^lib\\/" %default-include)
+         #:phases
+         ;; The package provides autoloads.
+         (modify-phases %standard-phases
+           (delete 'make-autoloads))))
+      (home-page "https://github.com/joaotavora/sly")
+      (synopsis "Sylvester the Cat's Common Lisp IDE")
+      (description
+       "SLY is Sylvester the Cat's Common Lisp IDE.  SLY is a fork of SLIME, and
+contains the following improvements over it:
+
+@enumerate
+@item Completely redesigned REPL based on Emacs's own full-featured
+  @code{comint.el}
+@item Live code annotations via a new @code{sly-stickers} contrib
+@item Consistent interactive button interface.  Everything can be copied to
+  the REPL.
+@item Multiple inspectors with independent history
+@item Regexp-capable @code{M-x sly-apropos}
+@item Contribs are first class SLY citizens and enabled by default
+@item Use ASDF to loads contribs on demand.
+@end enumerate
+
+SLY tracks SLIME's bugfixes and all its familar features (debugger, inspector,
+xref, etc...) are still available, but with better integration.")
+      (license license:gpl3+))))
+
 (define-public emacs-lua-mode
   (let ((commit "652e299cb967fccca827dda381d61a9c144d97de")
         (revision "1"))