1 Installation

• GNU ELPA package
• Manual installation

M-x package-refresh-contents
M-x package-install

cd ~/.emacs.d

# Create a directory for manually-installed packages
mkdir manual-packages

# Go to the new directory
cd manual-packages

# Clone this repo, naming it "ef-themes"
git clone https://git.sr.ht/~protesilaos/ef-themes ef-themes

;; Make Elisp files in that directory available to the user.
(add-to-list 'load-path "~/.emacs.d/manual-packages/ef-themes")

2 Sample configuration

;; Make customisations that affect Emacs faces BEFORE loading a theme
;; (any change needs a theme re-load to take effect).
(require 'ef-themes)

;; If you like two specific themes and want to switch between them, you
;; can specify them in `ef-themes-to-toggle' and then invoke the command
;; `ef-themes-toggle'.  All the themes are included in the variable
;; `ef-themes-collection'.
(setq ef-themes-to-toggle '(ef-summer ef-winter))

(setq ef-themes-headings ; read the manual's entry or the doc string
      '((0 variable-pitch light 1.9)
        (1 variable-pitch light 1.8)
        (2 variable-pitch regular 1.7)
        (3 variable-pitch regular 1.6)
        (4 variable-pitch regular 1.5)
        (5 variable-pitch 1.4) ; absence of weight means `bold'
        (6 variable-pitch 1.3)
        (7 variable-pitch 1.2)
        (t variable-pitch 1.1)))

;; They are nil by default...
(setq ef-themes-mixed-fonts t
      ef-themes-variable-pitch-ui t)

;; Disable all other themes to avoid awkward blending:
(mapc #'disable-theme custom-enabled-themes)

;; Load the theme of choice:
(load-theme 'ef-summer :no-confirm)

;; OR use this to load the theme which also calls `ef-themes-post-load-hook':
(ef-themes-select 'ef-summer)

;; The themes we provide are recorded in the `ef-themes-dark-themes',
;; `ef-themes-light-themes'.

;; We also provide these commands, but do not assign them to any key:
;;
;; - `ef-themes-toggle'
;; - `ef-themes-select'
;; - `ef-themes-select-dark'
;; - `ef-themes-select-light'
;; - `ef-themes-load-random'
;; - `ef-themes-preview-colors'
;; - `ef-themes-preview-colors-current'

3 Themes for red-green or blue-yellow color deficiency

Most items in the ef-themes collection are designed to use colors from many parts of the spectrum. Those items generally are inappropriate for users with red-green or blue-yellow color deficiency (deuteranopia and tritanopia, respectively). We do, nonetheless, provide light and dark themes that are optimized for such cases. The names of the themes are:

• ef-deuteranopia-dark and ef-deuteranopia-light for red-green color deficiency;
• ef-tritanopia-dark and ef-tritanopia-light for blue-yellow color deficiency;

Loading a theme.

4 Customization options

The ef-themes provide a limited set of user options which tweak secondary aspects of the theme. No option affects coloration or the overall character of each theme (Principles).

All customizations need to be evaluated before loading a theme. Any change thus requires a theme re-load (Loading a theme).

• Disable other themes
• Toggle between two Ef themes
• Enable mixed fonts
• UI typeface
• Option for headings
• Palette overrides
• Stylistic variants using palette overrides

(setq ef-themes-headings
      '((1 light variable-pitch 1.5)
        (2 regular 1.3)
        (3 1.1)
        (agenda-date 1.3)
        (agenda-structure variable-pitch light 1.8)
        (t variable-pitch)))

(semibold)
(variable-pitch semibold)
(variable-pitch semibold 1.3)
(variable-pitch semibold (height 1.3))   ; same as above
(variable-pitch semibold (height . 1.3)) ; same as above

(setq ef-themes-headings
      '((1 light variable-pitch 1.5)
        (2 regular 1.3)
        (3 1.1)
        (t variable-pitch)))

(setq ef-themes-headings
      '((1 . t)           ; keep the default style
        (2 variable-pitch 1.2)
        (t variable-pitch))) ; style for all unspecified headings

(setq ef-themes-headings
      '((1 variable-pitch 1.6)
        (2 1.3)
        (t . t))) ; default style for all unspecified levels

(defconst ef-summer-palette
  '(
;;; Basic values

    (bg-main     "#fff2f3")
    (fg-main     "#4f4073")
    (bg-dim      "#f2e4ea")
    (fg-dim      "#786e74")
    (bg-alt      "#efd3e4")
    (fg-alt      "#af4988")

    (bg-active   "#cfb3c4")
    (bg-inactive "#f7ebee")

;;; Basic hues for foreground values

    (red             "#d3303a")
    (red-warmer      "#e00033")
    (red-cooler      "#d50f7f")
    (red-faint       "#c24552")

    ;; ...

    (blue            "#375ce6")
    (blue-warmer     "#5250ef")
    (blue-cooler     "#065fff")
    (blue-faint      "#6060d0")

    ;; ...

;;; Mappings

    ;; ...

;;;; Code mappings

    (builtin magenta)
    (comment yellow-faint)
    (constant red-cooler)
    (fnname magenta-warmer)
    (keyword magenta-cooler)
    (preprocessor green-warmer)
    (docstring cyan-faint)
    (string yellow-warmer)
    (type cyan-warmer)
    (variable blue-warmer)

    ;; ...
    ))

(setq ef-summer-palette-overrides
      '((cursor "#ef9050")
        (comment red-faint)
        (keyword cyan-cooler)))

;; These overrides are common to all Ef themes.  We also provide
;; theme-specific options, such as `ef-summer-palette-overrides'.
;;
;; In general, the theme-specific overrides are better for overriding
;; color values, such as redefining what `blue-faint' looks like.  The
;; common overrides are best used for changes to semantic color
;; mappings, as we show below.
;;
;; Use the command `ef-themes-preview-colors' or its variant
;; `ef-themes-preview-colors-current' to review the available palette
;; entries.


;; Use an intense background combined with the theme's intense
;; foreground.
(setq ef-themes-common-palette-overrides
      '((bg-region bg-magenta-intense)
        (fg-region fg-intense)))

;; Same idea as above, but less pronounced.
(setq ef-themes-common-palette-overrides
      '((bg-region bg-magenta-subtle)
        (fg-region fg-main)))

5 Loading a theme

Emacs can load and maintain enabled multiple themes at once. This typically leads to awkward styling and weird combinations. The theme looks broken and the designer’s intent is misunderstood. Before loading one of the ef-themes, the user is encouraged to disable all others (Disable other themes):

(mapc #'disable-theme custom-enabled-themes)

Then load the theme of choice. For example:

(load-theme 'ef-summer :no-confirm)

The ‘:no-confirm’ is optional. It simply skips the step where Emacs asks the user whether they are sure about loading the theme.

Consider adding code like the above to the user configuration file, such as ‘init.el’.

NOTE: If you plan on using the ef-themes-post-load-hook, the above method will not work. Instead, you need to load the theme from your init file with ef-themes-select (Commands to load an Ef theme). For example:

;; like `load-theme' but also call `ef-themes-post-load-hook'
(ef-themes-select 'ef-summer)

6 Commands to load an Ef theme

The commands mentioned herein call ef-themes-post-load-hook at the end. This is for advanced users who want to trigger some code after an Ef theme is loaded (Use colors from the active Ef theme). The same goes for ef-themes-toggle (Toggle between two Ef themes).

A theme-agnostic hook for theme loading.

The themes can also be loaded interactively. The command ef-themes-select (call it with ‘M-x’) prompts with minibuffer completion for a theme among the collection of items we provide. It then loads the selected theme. Internally, ef-themes-select takes care to disable any other Ef theme, though it does not disable other themes. This is by design to let users maintain theme blending when they want it. Otherwise, the user is encouraged to disable all other themes and stick with the Ef themes:

(mapc #'disable-theme custom-enabled-themes)

The ef-themes-select command can also be called from the user’s init file to load the theme of their choice (Loading a theme). For example:

;; like `load-theme' but also call `ef-themes-post-load-hook'
(ef-themes-select 'ef-summer)

When ef-themes-select is called with a prefix argument (‘C-u’ by default), it first produces a minibuffer prompt to limit the choice to dark or light themes, and then shows only the relevant subset.

Instead of calling ef-themese-select with a prefix argument, users can invoke either of ef-themes-select-dark, ef-themes-select-light commands. Those behave the same as ef-themes-select except they limit the interaction to dark or light Ef themes, respectively.

The command ef-themes-load-random is for those who appreciate a bit of serendipity in their life. When call interactively with ‘M-x’ it loads a random theme from the Ef themes collection. The selection excludes the current Ef theme. Calling ef-themes-load-random with a prefix argument (‘C-u’ by default) makes the command limit the random selection to either light or dark themes. The user is prompted at the minibuffer to pick among the two sets.

The ef-themes-load-random can also be called from Lisp (e.g. from the ‘init.el’ file):

(ef-themes-load-random)

This has the effect of loading an Ef theme at random, as described above. It is also possible to limit the set with either of those:

;; Load a light Ef theme at random
(ef-themes-load-random 'light)

;; Load a dark Ef theme at random
(ef-themes-load-random 'dark)

The author of the Ef themes likes to check with their operating system to determine if the desktop environment outside of Emacs is using a light/dark theme and then loads an appropriate Ef theme at random:

;; Check GNOME's gsettings database for the dark theme preference.  If
;; it is enabled, we want to load a dark Ef theme at random.  Otherwise
;; we load a random light theme.
(if (string-match-p
     "dark"
     (shell-command-to-string "gsettings get org.gnome.desktop.interface color-scheme"))
    (ef-themes-load-random 'dark)
  (ef-themes-load-random 'light))

7 Preview theme colors

The command ef-themes-preview-colors uses minibuffer completion to select an item from the Ef themes and then produces a buffer with previews of its color palette entries. The buffer has a naming scheme which reflects the given choice, like ‘ef-summer-preview-colors’ for the ef-summer theme.

The command ef-themes-preview-colors-current skips the minibuffer selection process and just produces a preview for the current Ef theme.

When called with a prefix argument (‘C-u’ with the default key bindings), these commands will show a preview of the palette’s semantic color mappings instead of the named colors.

Aliases for those commands are ef-themes-list-colors and ef-themes-list-colors-current.

Overrides to color values are reflected in the buffers produced by the aforementioned commands (Palette overrides).

Each row shows a foreground and background coloration using the underlying value it references. For example a line with ‘#d3303a’ (a shade of red) will show red text followed by a stripe with that same color as a backdrop.

The name of the buffer describes the given Ef theme and what the contents are, such as ‘*ef-summer-list-colors*’ for named colors and ‘*ef-summer-list-mappings*’ for the semantic color mappings.

8 Use colors from the active Ef theme

Advanced users may want to call color variables from the palette of the active Ef theme. The macro ef-themes-with-colors supplies those to any form called inside of it. For example:

(ef-themes-with-colors
  (list bg-main fg-main bg-mode-line cursor))
;; => ("#fff2f3" "#5f456f" "#ffa4dc" "#cf0090")

The above return value is for ‘ef-summer’ when that is the active Ef theme. Switching to another theme and evaluating this code anew will give us the relevant results for that theme. The same with ‘ef-winter’:

(ef-themes-with-colors
  (list bg-main fg-main bg-mode-line cursor))
;; => ("#0f0b15" "#b8c6d5" "#5f1f5f" "#ff6ff0")

Do-It-Yourself customizations.

The palette of each Ef theme is considered stable. No removals shall be made. Though please note that some tweaks to individual hues or color mapping are still possible. At any rate, we will not outright break any code that uses ef-themes-with-colors.

9 Do-It-Yourself customizations

This section documents how the user can further tweak the Ef themes to their liking.

• Get a single color from the palette
• The general approach to advanced DIY changes
• A theme-agnostic hook for theme loading
• Do not extend the region background
• Add support for hl-todo
• Make the mode line like the modus-themes default
• Configure bold and italic faces
• Tweak ‘org-modern’ timestamps
• Tweak goto-address-mode faces

;; Here we show the recursion of palette mappings.  In general, it is
;; better for the user to specify named colors to avoid possible
;; confusion with their configuration, though those still work as
;; expected.
(setq ef-themes-common-palette-overrides
      '((cursor red)
        (prompt cursor)
        (variable prompt)))

;; Ignore the overrides and get the original value.
(ef-themes-get-color-value 'variable)
;; => "#5250ef"

;; Read from the overrides and deal with any recursion to find the
;; underlying value.
(ef-themes-get-color-value 'variable :overrides)
;; => "#d3303a"

(defun my-ef-themes-custom-faces ()
  "My customizations on top of the Ef themes.
This function is added to the `ef-themes-post-load-hook'."
  (ef-themes-with-colors
    (custom-set-faces
     ;; These are the default specifications
     `(font-lock-comment-face ((,c :inherit italic :foreground ,comment)))
     `(font-lock-variable-name-face ((,c :foreground ,variable))))))

;; Using the hook lets our changes persist when we use the commands
;; `ef-themes-toggle', `ef-themes-select', and `ef-themes-load-random'.
(add-hook 'ef-themes-post-load-hook #'my-ef-themes-custom-faces)

(defun my-ef-themes-custom-faces ()
  "My customizations on top of the Ef themes.
This function is added to the `ef-themes-post-load-hook'."
  (ef-themes-with-colors
    (custom-set-faces
     `(font-lock-comment-face ((,c :foreground ,variable)))
     `(font-lock-variable-name-face ((,c :inherit italic :foreground ,comment))))))

;; Using the hook lets our changes persist when we use the commands
;; `ef-themes-toggle', `ef-themes-select', and `ef-themes-load-random'.
(add-hook 'ef-themes-post-load-hook #'my-ef-themes-custom-faces)

(defun my-ef-themes-custom-faces ()
  "My customizations on top of the Ef themes.
This function is added to the `ef-themes-post-load-hook'."
  (ef-themes-with-colors
    (custom-set-faces
     `(font-lock-comment-face ((,c :foreground ,variable)))
     `(font-lock-variable-name-face ((,c :inherit italic :foreground ,comment))))))

;; Using the hook lets our changes persist when we use the commands
;; `ef-themes-toggle', `ef-themes-select', and `ef-themes-load-random'.
(add-hook 'ef-themes-post-load-hook #'my-ef-themes-custom-faces)

;; Load the theme and run `ef-themes-post-load-hook'
(ef-themes-select 'ef-summer) ; Instead of (load-theme 'ef-summer :no-confirm)

(defvar after-enable-theme-hook nil
   "Normal hook run after enabling a theme.")

(defun run-after-enable-theme-hook (&rest _args)
   "Run `after-enable-theme-hook'."
   (run-hooks 'after-enable-theme-hook))

(advice-add 'enable-theme :after #'run-after-enable-theme-hook)

;; Do not extend `region' background past the end of the line.
(custom-set-faces
 '(region ((t :extend nil))))

(defun my-ef-themes-hl-todo-faces ()
  "Configure `hl-todo-keyword-faces' with Ef themes colors.
The exact color values are taken from the active Ef theme."
  (ef-themes-with-colors
    (setq hl-todo-keyword-faces
          `(("HOLD" . ,yellow)
            ("TODO" . ,red)
            ("NEXT" . ,blue)
            ("THEM" . ,magenta)
            ("PROG" . ,cyan-warmer)
            ("OKAY" . ,green-warmer)
            ("DONT" . ,yellow-warmer)
            ("FAIL" . ,red-warmer)
            ("BUG" . ,red-warmer)
            ("DONE" . ,green)
            ("NOTE" . ,blue-warmer)
            ("KLUDGE" . ,cyan)
            ("HACK" . ,cyan)
            ("TEMP" . ,red)
            ("FIXME" . ,red-warmer)
            ("XXX+" . ,red-warmer)
            ("REVIEW" . ,red)
            ("DEPRECATED" . ,yellow)))))

(add-hook 'ef-themes-post-load-hook #'my-ef-themes-hl-todo-faces)

(defun my-ef-themes-mode-line ()
  "Tweak the style of the mode lines."
  (ef-themes-with-colors
    (custom-set-faces
     `(mode-line ((,c :background ,bg-active :foreground ,fg-main :box (:line-width 1 :color ,fg-dim))))
     `(mode-line-inactive ((,c :box (:line-width 1 :color ,bg-active)))))))

(add-hook 'ef-themes-post-load-hook #'my-ef-themes-mode-line)

(mode-line ((,c :background ,bg-active :foreground ,fg-main :box (:line-width 1 :color ,fg-dim))))

`(mode-line ((,c :background ,bg-mode-line :foreground ,fg-mode-line :box (:line-width 1 :color ,fg-dim))))

(set-face-attribute 'default nil :family "Fira Code" :height 110)
(set-face-attribute 'italic nil :family "Hack")

(set-face-attribute 'default nil :family "Source Code Pro" :height 110 :weight 'light)
(set-face-attribute 'bold nil :weight 'semibold)

(set-face-attribute 'italic nil :family 'unspecified)

(setq org-modern-timestamp nil)

(setq goto-address-url-face 'link
      goto-address-url-mouse-face 'highlight
      goto-address-mail-face 'link
      goto-address-mail-mouse-face 'highlight)

10 Faces defined by the Ef themes

The themes define some faces to make it possible to achieve consistency between various groups of faces. For example, all “marks for selection” use the ef-themes-mark-select face. If, say, the user wants to edit this face to include an underline, the change will apply to lots of packages, like Dired, Trashed, Ibuffer.

Do-It-Yourself customizations.

All the faces defined by the themes:

• ef-themes-fixed-pitch
• ef-themes-heading-0
• ef-themes-heading-1
• ef-themes-heading-2
• ef-themes-heading-3
• ef-themes-heading-4
• ef-themes-heading-5
• ef-themes-heading-6
• ef-themes-heading-7
• ef-themes-heading-8
• ef-themes-key-binding
• ef-themes-mark-delete
• ef-themes-mark-other
• ef-themes-mark-select
• ef-themes-ui-variable-pitch
• ef-themes-underline-error
• ef-themes-underline-info
• ef-themes-underline-warning

11 Supported packages or face groups

The ef-themes will only ever support a curated list of packages based on my judgement (Packages that are hard to support). Nevertheless, the list of explicitly or implicitly supported packages already covers everything most users need.

• Explicitly supported packages or face groups
• Implicitly supported packages or face groups
• Packages that are hard to support

12 Principles

• No customization options that affect the coloration of individual components like a heading or a construct in the code! Either you like them or you don’t. To change the attributes of individual faces, it is better to do it in user-level configurations (Do-It-Yourself customizations). Report the issue and I can help with the relevant code. Providing customizations that refashion a wide set of themes is not maintainable due to the sheer magnitude of all the possible combinations.
• Curated list of supported faces (Supported packages or face groups). I will only cover what I use and/or know is in a well-maintained state. Styling a package requires a lot of work as one must (i) know how the package works, (ii) see all its interfaces in action, (iii) make sure that the color combinations look consistent with the rest of the theme.
• Minimum contrast ratio of 4.5:1 against the main background (WCAG AA standard). Emphasis on “minimum” as themes can always have more contrast. Use my modus-themes package (also built into Emacs 28 or higher) if you need top-class accessibility in this regard.
• If two elements are functionally distinct, the theme must render this fact conspicuous (Let Org tags inherit the heading color?).
• Respect the decisions of the upstream package provider or, else, do not make weird things with faces. I only override the design choice of a package if it is absolutely necessary and then still try to conform with the principle of least surprise.
• When there is an inescapable trade-off between usability and aesthetics, I will always opt for the former, without prejudice to the aforementioned principles.

• Minimal VS maximal scope

13 FAQ

• What does Ef mean?
• Will you stop using the modus-themes?
• Then why develop the ef-themes?
• Difference between ef-themes and modus-themes?
• Too many options, what should I do?
• Let Org tags inherit the heading color?
• Add something like Nord or Zenburn?

14 Acknowledgements

This project is meant to be a collective effort. Every bit of help matters.

Author/maintainer

Protesilaos Stavrou.

Contributions to code or the manual

Alex Lu, Christopher League, Eshel Yaron, Gautier Ponsinet, Oleksii (Alex) Koval, Philip Kaludercic, Ryan Kaskel, Walheimat.

Ideas and/or user feedback

Adam Porter (alphapapa), Alan Schmitt, Amo DelBello, Anthony Chavez, Benjamin (zealotrush), Bruno Boal, Daniel Mendler, Federico Stilman, Iris Garcia, Jean-Philippe Gagné Guay, Jonas Collberg, Saša Janiška, Spike-Leung, Steve Molitor, Summer Emacs, Sven Seebeck, Thanos Apollo, Zoltán Király, goldfita, marasenna, newhallroad.

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3. COPYING IN QUANTITY

   If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
   15. Preserve any Warranty Disclaimers.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.

   You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

   However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

   Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

   Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See https://www.gnu.org/licenses/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

11. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

    with the Invariant Sections being list their titles, with
    the Front-Cover Texts being list, and with the Back-Cover Texts
    being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

16 Indices

• Function index
• Variable index
• Concept index