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-# -*- mode: org; coding: utf-8-unix -*-
-#+STARTUP: showall
-#+OPTIONS: num:nil
-#+TITLE: Org-Drill
-#+AUTHOR: Paul Sexton
-
-* Synopsis
-
-
-Org-Drill is an extension for [[http://www.gnu.org/software/emacs/][GNU Emacs]] [[http://orgmode.org/][Org mode]]. Org-Drill uses a [[http://en.wikipedia.org/wiki/Spaced_repetition][spaced
-repetition]] algorithm to conduct interactive "drill sessions", using org files
-as sources of facts to be memorised. Each topic is treated as a "flash
-card". The material to be remembered is presented to the student in random
-order. The student rates his or her recall of each item, and this information
-is fed back to =org-learn= to schedule the item for later revision.
-
-Each drill session can be restricted to topics in the current buffer
-(default), one or several files, all agenda files, or a subtree. A single
-topic can also be drilled.
-
-Different "topic types" can be defined, which present their information to the
-student in different ways.
-
-For more on the spaced repetition algorithm, and examples of other programs
-that use it, see:
-- [[http://supermemo.com/index.htm][SuperMemo]] (see descriptions of the SM2, SM5 and SM8 algorithms)
-- [[http://ichi2.net/anki/][Anki]]
-- [[http://mnemosyne-proj.org/index.php][Mnemosyne]]
-
-Org-Drill comes bundled with Org mode, in the "contrib" directory. Org-Drill
-also has its own repository, which is updated more regularly than the bundled
-version. The repository is at:
-
-http://bitbucket.org/eeeickythump/org-drill
-
-
-* Installation
-
-
-Put the following in your =.emacs=. You will also need to make sure that Org's
-"contrib/lisp" directory is in the emacs load-path.
-
-#+BEGIN_EXAMPLE
-(require 'org-drill)
-#+END_EXAMPLE
-
-
-
-* Demonstration
-
-
-Load the file [[file:spanish.org][spanish.org]]. Press =M-x= and run the function =org-drill=. Follow
-the prompts at the bottom of the screen.
-
-When the drill finishes, you can look at =spanish.org= to get some idea of how
-drill topics are written.
-
-
-* Writing the questions
-
-
-Org-Drill uses org mode topics as 'drill items'. To be used as a drill item,
-the topic must have a tag that matches the value of
-=org-drill-question-tag=. This is =:drill:= by default. Any other org topics
-will be ignored.
-
-Drill items can have other drill items as children. When a drill item is being
-tested, the contents of any child drill items will be hidden.
-
-You don't need to schedule the topics initially.  Unscheduled items are
-considered to be 'new' and ready for memorisation.
-
-How should 'drill topics' be structured? Any org topic is a legal drill topic
--- it will simply be shown with all subheadings collapsed, so that only the
-material beneath the main item heading is visible. After pressing a key, any
-hidden subheadings will be revealed, and you will be asked to rate your
-"recall" of the item.
-
-This will be adequate for some items, but usually you will want to write items
-where you have more control over what information is hidden from the user for
-recall purposes. For this reason, some other card types are defined, including:
-- [[Two-sided cards]]
-- [[Multi-sided cards]]
-- [[Multi-cloze cards]]
-- [[User-defined card types]]
-
-*A note about comments:* In org mode, comment lines start with '#'. The rest of
-the line is ignored by Org (apart from some special cases). You may sometimes
-want to put material in comments which you do not want to see when you are
-being tested on the item. For this reason, comments are always rendered
-invisible while items are being tested.
-
-
-** Simple topics
-
-
-The simplest drill topic has no special structure. When such a topic is
-presented during a drill session, any subheadings are "collapsed" with their
-contents hidden. So, you could include the question as text beneath the main
-heading, and the answer within a subheading. For example:
-
-#+BEGIN_EXAMPLE
-* Item                                   :drill:
-What is the capital city of Estonia?
-
-** The Answer
-Tallinn.
-#+END_EXAMPLE
-
-When this item is presented for review, the text beneath the main heading will
-be visible, but the contents of the subheading ("The Answer") will be hidden.
-
-
-** Cloze deletion
-
-
-Cloze deletion can be used in any drill topic regardless of whether it is
-otherwise 'simple', or is one of the specialised topic types discussed
-below. To use cloze deletion, one or more parts of the body of the topic is
-marked as /cloze text/ by surrounding it with single square brackets, [like
-so]. When the topic is presented for review, the text within square brackets
-will be obscured. The text is then revealed after the user presses a key. For
-example:
-
-
-#+BEGIN_EXAMPLE
-* Item                                   :drill:
-The capital city of Estonia is [Tallinn].
-#+END_EXAMPLE
-
-During review, the user will see:
-
-#+BEGIN_QUOTE
-The capital city of Estonia is @<font style="background-color: blue;" color="cyan">
-@<tt>[...]@</tt>@</font>.
-#+END_QUOTE
-
-When the user presses a key, the text "Tallinn" will become visible.
-
-
-** Clozed text hints
-
-
-Clozed text can contain a "hint" about the answer. If the text surrounded
-by single square brackets contains a `|' character (vertical bar), all text
-after that character is treated as a hint. During testing, the hint text will
-be visible when the rest of the text is hidden, and invisible when the rest of
-the text is visible.
-
-Example:
-
-#+BEGIN_EXAMPLE
-Type 1 hypersensitivity reactions are mediated by [immunoglobulin E|molecule]
-and [mast cells|cell type].
-#+END_EXAMPLE
-
-#+BEGIN_QUOTE
-Type 1 hypersensitivity reactions are mediated by
-@<font style="background-color: blue;" color="cyan">
-@<tt>[molecule...]@</tt>@</font>
-and @<font style="background-color: blue;" color="cyan">
-@<tt>[cell type...]@</tt>@</font>.
-#+END_QUOTE
-
-
-** Two-sided cards
-# <<Two-sided cards>>
-
-The remaining topic types all use the topic property, =DRILL_CARD_TYPE=. This
-property tells =org-drill= which function to use to present the topic during
-review. If this property has the value =twosided= then the topic is treated as
-a "two sided card". When a two sided card is reviewed, /one of the first two/
-subheadings within the topic will be visible -- all other
-subheadings will be hidden.
-
-Two-sided cards are meant to emulate the type of flipcard where either side is
-useful as test material (for example, a card with a word in a foreign language
-on one side, and its translation on the other).
-
-A two sided card can have more than 2 subheadings, but all subheadings after
-the first two are considered as "notes" and will always be hidden during topic
-review.
-
-#+BEGIN_EXAMPLE
-* Noun                                               :drill:
-    :PROPERTIES:
-    :DRILL_CARD_TYPE: twosided
-    :END:
-
-Translate this word.
-
-** Spanish
-la mujer
-
-** English
-the woman
-
-** Example sentence
-¿Quién fue esa mujer?
-Who was that woman?
-#+END_EXAMPLE
-
-In this example, the user will be shown the main text -- "Translate this word"
--- and either 'la mujer', /or/ 'the woman', at random. The section 'Example
-sentence' will never be shown until after the user presses a key, because it is
-not one of the first two 'sides' of the topic.
-
-
-** Multi-sided cards
-# <<Multi-sided cards>>
-
-
-The =multisided= card type is similar to =twosided=, except that any
-subheading has a chance of being presented during the topic review. One
-subheading is always shown and all others are always hidden.
-
-#+BEGIN_EXAMPLE
-* Noun                                               :drill:
-    :PROPERTIES:
-    :DRILL_CARD_TYPE: multisided
-    :END:
-
-Translate.
-
-** Spanish
-la mesa
-
-** English
-the table
-
-** Picture
-[[file:table.jpg][PICTURE]]
-#+END_EXAMPLE
-
-The user will be shown the main text and either 'la mujer', /or/ 'the woman',
-/or/ a picture of a table.
-
-
-** Multi-cloze cards
-# <<Multi-cloze cards>>
-
-
-Often, you will wish to create cards out of sentences that express several
-facts, such as the following:
-
-#+BEGIN_EXAMPLE
-The capital city of New Zealand is Wellington, which is located in the
-North Island and has a population of about 400,000.
-#+END_EXAMPLE
-
-There is more than one fact in this statement -- you could create a single
-'simple' card with all the facts marked as cloze text, like so:
-
-#+BEGIN_EXAMPLE
-The capital city of [New Zealand] is [Wellington], which is located in
-the [North|North/South] Island and has a population of about [400,000].
-#+END_EXAMPLE
-
-But this card will be difficult to remember. If you get just one of the 4
-hidden facts wrong, you will fail the card. A card like this is likely to
-become a [[leeches][leech]].
-
-A better way to express all these facts using 'simple' cards is to create
-several cards, with one fact per card. You might end up with something
-like this:
-
-#+BEGIN_EXAMPLE
-* Fact
-The capital city of [New Zealand] is Wellington, which has a population of
-about 400,000.
-
-* Fact
-The capital city of New Zealand is [Wellington], which has a population of
-about 400,000.
-
-* Fact
-The capital city of New Zealand is Wellington, which has a population of
-about [400,000].
-
-* Fact
-The capital city of [New Zealand] is Wellington, which is located in the
-the North Island.
-
-* Fact
-The capital city of New Zealand is [Wellington], which is located in
-the North Island.
-
-* Fact
-The capital city of New Zealand is Wellington, which is located in
-the [North|North/South] Island.
-#+END_EXAMPLE
-
-However, this is really cumbersome. Multicloze card types exist for this
-situation. Multicloze cards behave like 'simple' cards, except that when there
-is more than one area marked as cloze text, some but not all of the areas
-can be hidden. There are several types of predefined multicloze card:
-
-1. =hide1cloze= -- one of the marked areas is hidden during review; the others
-   all remain visible. The hidden text area is chosen randomly at each review.
-   (Note: this type used to be called 'multicloze', and that card type is
-   retained as a synonym for 'hide1cloze'.)
-2. =show1cloze= -- only one of the marked areas is visible during review; all
-   the others are hidden. The hidden text area is chosen randomly at each
-   review.
-3. =hide2cloze= -- like hide1cloze, but 2 marked pieces of text will be hidden,
-   and the rest will be visible.
-4. =show2cloze= -- like show1cloze, but 2 marked pieces of text will be visible,
-   the rest are hidden.
-
-There are also some types of multicloze card where some pieces have an
-increased or decreased chance of being hidden. These are intended for use when
-studying languages: generally it is easy to translate a foreign-language
-sentence into your own language if you have met it before, but it is much
-harder to translate in the other direction. Therefore, you will want to test
-the harder direction more often.
-5. =hide1_firstmore= -- only one of the marked pieces of text will be
-   hidden. 75% of the time (guaranteed), the /first/ piece is hidden; the rest
-   of the time, one of the other pieces is randomly hidden.
-6. =show1_firstless= -- only one of the marked pieces of text will be
-   visible. Only 25% of the time (guaranteed) will the /first/ piece will be
-   visible; the rest of the time, one of the other pieces is randomly visible.
-7. =show1_lastmore= -- only one of the marked pieces of text will be
-   visible. 75% of the time (guaranteed), the /last/ piece will be visible;
-   the rest of the time, one of the other pieces is randomly visible.
-
-So, for the above example, we can actually use the original 'bad' simple card,
-but change its card type to 'hide1cloze'. Each time the card is presented for
-review, one of 'New Zealand', 'Wellington', 'the South Island' or '400,000'
-will be hidden.
-
-#+BEGIN_EXAMPLE
-* Fact
-  :PROPERTIES:
-  :DRILL_CARD_TYPE: hide1cloze
-  :END:
-
-The capital city of [New Zealand] is [Wellington], which is located in
-the [North|North/South] Island and has a population of about [400,000].
-#+END_EXAMPLE
-
-
-** User-defined card types
-# <<User-defined card types>>
-
-
-Finally, you can write your own emacs lisp functions to define new kinds of
-topics. Any new topic type will need to be added to
-=org-drill-card-type-alist=, and cards using that topic type will need to have
-it as the value of their =DRILL_CARD_TYPE= property. For examples, see the
-functions at the end of org-drill.el -- these include:
-- =org-drill-present-verb-conjugation=, which implements the 'conjugate'
-  card type. This asks the user to conjugate a verb in a particular tense. It
-  demonstrates how the appearance of an entry can be completely altered during
-  a drill session, both during testing and during the display of the answer.
-- =org-drill-present-translate-number=, which uses a third-party emacs lisp
-  library ([[http://www.emacswiki.org/emacs/spell-number.el][spell-number.el]]) to prompt the user to translate random numbers
-  to and from any language recognised by that library.
-- =org-drill-present-spanish-verb=, which defines the new topic type
-  =spanish_verb=. This illustrates how a function can control which of an
-  item's subheadings are visible during the drill session.
-
-See the file [[file:spanish.org][spanish.org]] for a full set of example material, including examples
-of all the card types discussed above.
-
-
-** Empty cards
-
-
-If the body of a drill item is completely empty (ignoring properties and child
-items), then the item will be skipped during drill sessions. The purpose of
-this behaviour is to allow you to paste in 'skeletons' of complex items, then
-fill in missing information later. For example, you may wish to include an
-empty drill item for each tense of a newly learned verb, then paste in the
-actual conjugation later as you learn each tense.
-
-Note that if an item is empty, any child drill items will *not* be ignored,
-unless they are empty as well.
-
-If you have an item with an empty body, but still want it to be included in a
-drill session, put a brief comment ('# ...')  in the item body.
-
-
-* Running the drill session
-
-
-Start a drill session with =M-x org-drill=. By default, this includes all
-non-hidden topics in the current buffer. =org-drill= takes an optional
-argument, SCOPE, which allows it to take drill items from other
-sources. See [[scope][below]] for details.
-
-During a drill session, you will be presented with each item, then asked to
-rate your recall of it by pressing a key between 0 and 5. The meaning of these
-numbers is (taken from =org-learn=):
-
-| Quality | SuperMemo label | Fail? | Meaning                                              |
-|---------+-----------------+-------+------------------------------------------------------|
-|       0 | NULL            | Yes   | Wrong, and the answer is unfamiliar when you see it. |
-|       1 | BAD             | Yes   | Wrong answer.                                        |
-|       2 | FAIL            | Yes   | Almost, but not quite correct.                       |
-|       3 | PASS            | No    | Correct answer, but with much effort.                |
-|       4 | GOOD            | No    | Correct answer, with a little thought.               |
-|       5 | BRIGHT          | No    | Correct answer, effortless.                          |
-
-You can press '?'  at the prompt if you have trouble remembering what the
-numbers 0--5 signify.
-
-At any time you can press 'q' to finish the drill early (your progress up to
-that point will be saved), 's' to skip the current item without viewing the
-answer, or 'e' to escape from the drill and jump to the current topic for
-editing (again, your progress up to that point will be saved).
-
-After exiting the drill session with 'e' or 'q', you can resume where you left
-off, using the command =org-drill-resume=. This will return you to the item
-that you were viewing when you left the session. For example, if you are shown
-an item and realise that it is poorly formulated, or contains an error, you can
-press 'e' to leave the drill, then correct the item, then press
-=M-x org-drill-resume= and continue where you left off.
-
-Note that 'drastic' edits, such as deleting or moving items, can sometimes
-cause Org-Drill to "lose its place" in the file, preventing it from
-successfully resuming the session. In that case you will need to start a new
-session.
-
-
-* Multiple sequential drill sessions
-
-
-Org-Drill has to scan your entire item database each time you start a new drill
-session. This can be slow if you have a large item collection. If you have a
-large number of 'due' items and want to run a second drill session after
-finishing one session, you can use the command =org-drill-again= to run a new
-drill session that draws from the pool of remaining due items that were not
-tested during the previous session, without re-scanning the item collection.
-
-Also note that if you run =org-drill-resume= and you have actually finished the
-drill session, you will be asked whether you want to start another drill
-session without re-scanning (as if you had run =org-drill-again=).
-
-
-* Cram mode
-
-
-There are some situations, such as before an exam, where you will want to
-revise all of your cards regardless of when they are next due for review.
-
-To do this, run a /cram session/ with the =org-drill-cram= command (=M-x
-org-drill-cram RET=). This works the same as a normal drill session, except
-that all items are considered due for review unless you reviewed them within
-the last 12 hours (you can change the number of hours by customising the
-variable =org-drill-cram-hours=).
-
-
-* Leeches
-# <<leeches>>
-
-From the Anki website, http://ichi2.net/anki/wiki/Leeches:
-
-#+BEGIN_QUOTE
-Leeches are cards that you keep on forgetting. Because they require so many
-reviews, they take up a lot more of your time than other cards.
-#+END_QUOTE
-
-Like Anki, Org-Drill defines leeches as cards that you have "failed" many
-times. The number of times an item must be failed before it is considered a
-leech is set by the variable =org-drill-leech-failure-threshold= (15 by
-default). When you fail to remember an item more than this many times, the item
-will be given the =:leech:= tag.
-
-Leech items can be handled in one of three ways. You can choose how Org-Drill
-handles leeches by setting the variable =org-drill-leech-method= to one of the
-following values:
-- nil :: Leech items are tagged with the =leech= tag, but otherwise treated the
-         same as normal items.
-- skip :: Leech items are not included in drill sessions.
-- warn :: Leech items are still included in drill sessions, but a warning
-  message is printed when each leech item is presented.
-
-The best way to deal with a leech is either to delete it, or reformulate it so
-that it is easier to remember, for example by splitting it into more than one
-card.
-
-See [[http://www.supermemo.com/help/leech.htm][the SuperMemo website]] for more on leeches.
-
-
-* Customisation
-
-
-Org-Drill has several settings which you change using
-=M-x customize-group org-drill <RET>=. Alternatively you can change these
-settings by adding elisp code to your configuration file (=.emacs=).
-
-
-** Visual appearance of items during drill sessions
-
-
-If you want cloze-deleted text to show up in a special font within Org mode
-buffers, add this to your .emacs:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-use-visible-cloze-face-p t)
-#+END_EXAMPLE
-
-Item headings may contain information that "gives away" the answer to the item,
-either in the heading text or in tags. If you want item headings to be made
-invisible while each item is being tested, add:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-hide-item-headings-p t)
-#+END_EXAMPLE
-
-
-** Duration of drill sessions
-
-
-By default, a drill session will end when either 30 items have been
-successfully reviewed, or 20 minutes have passed. To change this behaviour, use
-the following settings.
-
-#+BEGIN_EXAMPLE
-(setq org-drill-maximum-items-per-session 40)
-(setq org-drill-maximum-duration 30)   ; 30 minutes
-#+END_EXAMPLE
-
-If either of these variables is set to nil, then item count or elapsed time
-will not count as reasons to end the session. If both variables are nil, the
-session will not end until /all/ outstanding items have been reviewed.
-
-
-** Saving buffers after drill sessions
-
-
-By default, you will be prompted to save all unsaved buffers at the end of a
-drill session. If you don't like this behaviour, use the following setting:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-save-buffers-after-drill-sessions-p nil)
-#+END_EXAMPLE
-
-
-** Sources of items for drill sessions (scope)
-# <<scope>>
-
-By default, Org-Drill gathers drill items from the current buffer only,
-ignoring any non-visible items. There may be times when you want Org-Drill to
-gather drill items from other sources. You can do this by changing the value of
-the variable =org-drill-scope=. Possible values are:
-
-- file :: The current buffer, ignoring hidden items. This is the default.
-- tree :: The subtree starting with the entry at the cursor. (Alternatively you
-          can use =M-x org=drill-tree= to run the drill session -- this will
-          behave the same as =org-drill= if 'tree' was used as the value of
-          SCOPE.)
-- file-no-restriction :: The current buffer, including both hidden and
-     non-hidden items.
-- file-with-archives :: The current buffer, and any archives associated with it.
-- agenda :: All agenda files.
-- agenda-with-archives :: All agenda files with any archive files associated
-     with them.
-- directory :: All files with the extension '.org' in the same directory as the
-               current file. (The current file will also be included if its
-               extension is .org)
-- (file1 file2 ...) :: A list of filenames. All files in the list will be
-     scanned.
-
-
-
-** Definition of old and overdue items
-
-
-Org-Drill prioritises /overdue/ items in each drill session, presenting them
-before other items are seen. Overdue items are defined in terms of how far in
-the past the item is scheduled for review. The threshold is defined in terms
-of a proportion rather than an absolute number of days. If days overdue is
-greater than
-
-: last-interval * (factor - 1)
-
-and is at least one day overdue, then the item is considered 'overdue'. The
-default factor is 1.2, meaning that the due date can overrun by 20% before the
-item is considered overdue.
-
-To change the factor that determines when items become overdue, use something
-like the following in your .emacs. Note that the value should never be less
-than 1.0.
-
-#+BEGIN_EXAMPLE
-(setq org-drill-overdue-interval-factor 1.1)
-#+END_EXAMPLE
-
-After prioritising overdue items, Org-Drill next prioritises /young/
-items. These are items which were recently learned (or relearned in the case of
-a failure), and which therefore have short inter-repetition intervals.
-"Recent" is defined as an inter-repetition interval less than a fixed number of
-days, rather than a number of repetitions. This ensures that more difficult
-items are reviewed more often than easier items before they stop being 'young'.
-
-The default definition of a young item is one with an inter-repetition interval
-of 10 days or less. To change this, use the following:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-days-before-old 7)
-#+END_EXAMPLE
-
-
-** Spaced repetition algorithm
-
-
-*** Choice of algorithm
-
-
-Org-Drill supports three different spaced repetition algorithms, all based on
-SuperMemo algorithms. These are:
-- [[http://www.supermemo.com/english/ol/sm2.htm][SM2]] :: an early algorithm, used in SuperMemo 2.0 (1988), which remains very
-  popular -- Anki and Mnemosyne, two of the most popular spaced repetition
-  programs, use SM2. This algorithm stores an 'ease factor' for each item,
-  which is modified each time you rate your recall of the item.
-- [[http://www.supermemo.com/english/ol/sm5.htm][SM5]] (default) :: used in SuperMemo 5.0 (1989). This algorithm uses 'ease
-     factors' but also uses a persistent, per-user 'matrix of optimal factors'
-     which is also modified after each item repetition.
-- Simple8 :: an experimental algorithm based on the [[http://www.supermemo.com/english/algsm8.htm][SM8]] algorithm. SM8 is used
-             in SuperMemo 8.0 (1998) and is almost identical to SM11 which is
-             used in SuperMemo 2002. Like SM5, it uses a matrix of optimal
-             factors. Simple8 differs from SM8 in that it does not adapt the
-             matrix to the individual user, though it does adapt each item's
-             'ease factor'.
-
-
-If you want Org-Drill to use the =SM2= algorithm, put the following in your
-=.emacs=:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-spaced-repetition-algorithm 'sm2)
-#+END_EXAMPLE
-
-
-*** Random variation of repetition intervals
-
-
-The intervals generated by the SM2 and SM5 algorithms are pretty
-deterministic. If you tend to add items in large, infrequent batches, the lack
-of variation in interval scheduling can lead to the problem of "lumpiness" --
-one day a large batch of items are due for review, the next there is almost
-nothing, a few days later another big pile of items is due.
-
-This problem can be ameliorated by adding some random "noise" to the interval
-scheduling algorithm. The author of SuperMemo actually recommends this approach
-for the SM5 algorithm, and Org-Drill's implementation uses [[http://www.supermemo.com/english/ol/sm5.htm][his code]].
-
-To enable random "noise" for item intervals, set the variable
-=org-drill-add-random-noise-to-intervals-p= to true by putting the following in
-your =.emacs=:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-add-random-noise-to-intervals-p t)
-#+END_EXAMPLE
-
-
-*** Adjustment for early or late review of items
-
-
-Reviewing items earlier or later than their scheduled review date may affect
-how soon the next review date should be scheduled. Code to make this adjustment
-is also presented on the SuperMemo website. It can be enabled with:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-adjust-intervals-for-early-and-late-repetitions-p t)
-#+END_EXAMPLE
-
-This will affect both early and late repetitions if the Simple8 algorithm is
-used. For the SM5 algorithm it will affect early repetitions only. It has no
-effect on the SM2 algorithm.
-
-
-*** Adjusting item difficulty globally
-
-
-The =learn fraction= is a global value which affects how quickly the intervals
-(times between each retest of an item) increase with successive repetitions,
-for /all/ items. The default value is 0.5, and this is the value used in
-SuperMemo. For some collections of information, you may find that you are
-reviewing items too often (they are too easy and the workload is too high), or
-too seldom (you are failing them too often). In these situations, it is
-possible to alter the learn fraction from its default in order to increase or
-decrease the frequency of repetition of items over time. Increasing the value
-will make the time intervals grow faster, and lowering it will make them grow
-more slowly. The table below shows the growth in intervals (in days) with some
-different values of the learn fraction (F). The table assumes that the item is
-successfully recalled each time, with an average quality of just under 4.
-
-
-| Repetition | F=0.3 | F=0.4 | *F=0.5* | F=0.6 | F=0.7 |
-|------------+-------+-------+---------+-------+-------|
-| 1st        |     2 |     2 |       2 |     2 |     2 |
-| 2nd        |     7 |     7 |       7 |     7 |     7 |
-| 5th        |    26 |    34 |      46 |    63 |    85 |
-| 10th       |    85 |   152 |     316 |   743 |  1942 |
-| 15th       |   233 |   501 |    1426 |  5471 | 27868 |
-
-To alter the learn fraction, put the following in your .emacs:
-
-#+BEGIN_EXAMPLE
-(setq org-drill-learn-fraction 0.45)   ; change the value as desired
-#+END_EXAMPLE
-
-
-** Per-file customisation settings
-# <<per-file settings>>
-
-Most of Org-Drill's customisation settings are safe as file-local
-variables. This means you can include a commented section like this at the end
-of your .org file to apply special settings when running a Drill session using
-that file:
-
-#+BEGIN_EXAMPLE
-# Local Variables:
-# org-drill-maximum-items-per-session:    50
-# org-drill-spaced-repetition-algorithm:  simple8
-# End:
-#+END_EXAMPLE
-
-You can achieve the same effect by including the settings in the 'mode line'
-(this must be the *first line* in the file), like so:
-
-#+BEGIN_EXAMPLE
-# -*- org-drill-maximum-items-per-session: 50; org-drill-spaced-repetition-algorithm: simple8 -*-
-#+END_EXAMPLE
-
-In either case you will need to save, close and re-open the file for the
-changes to take effect.
-
-
-* Coping with large collections
-
-
-If you keep all your items in a single file, it may eventually very large. The
-file will be slow to load, and Emacs may have trouble syntax-highlighting the
-file contents correctly.
-
-The easiest steps to solve this problem are:
-1. Move your file into its own dedicated directory.
-2. Divide the file into two or more smaller files.
-3. Within each file, set =org-drill-scope= to 'directory'. See
-   [[per-file settings]] above for instructions about how to do this.
-
-
-* Sharing, merging and synchronising item collections
-
-
-Every drill item is automatically given a persistent unique "ID" the first time
-it is seen by Org-Drill. This means that if two different people subsequently
-edit or reschedule that item, Org-Drill can still tell that it is the same
-item. This in turn means that collections of items can be shared and edited in
-a collaborative manner.
-
-There are two commands that are useful in this regard:
-1. =org-drill-strip-all-data= - this command deletes all user-specific
-   scheduling data from every item in the current collection. (It takes the
-   same optional 'scope' argument as =org-drill= to define which items will
-   be processed by the command). User-specific data includes scheduling dates,
-   ease factors, number of failures and repetitions, and so on. All items are
-   reset to 'new' status. This command is useful if you want to share your
-   item collection with someone else.
-2. =org-drill-merge-buffers= - When called from buffer A, it prompts you for
-   another buffer (B), which must also be loaded into Emacs. This command
-   imports all the user-specific scheduling data from buffer B into buffer A,
-   and deletes any such information in A. Matching items are identified by
-   their ID. Any items in B that do not exist in A are copied to A, in
-   the same hierarchical location if all the parent headings exist, otherwise
-   at the end of the buffer.
-
-An example scenario:
-
-Tim decides to learn Swedish using an item collection (=.org= file) made
-publically available by Jane.  (Before publishing it Jane used
-'org-drill-strip-all-data' to remove her personal scheduling data from the
-collection.)  A few weeks later, Jane updates her collection, adding new items
-and revising some old ones. Tim downloads the new collection and imports his
-progress from his copy of the old collection, using 'org-drill-merge-buffers',
-using the new collection as buffer A and the old one as buffer B. He can then
-discard the old copy. Any items HE added to HIS copy of the old collection
-(buffer B) will not be lost -- they will be appended to his copy of the new
-collection.
-
-Of course the sharing does not need to be 'public'. You and a friend might be
-learning a language or some other topic together. You each maintain a card
-collection. Periodically your friend sends you a copy of their collection --
-you run =org-drill-merge-buffers= on it, always using your own collection as
-buffer B so that your own scheduling progress is carried over. Other times you
-send your friend a copy of your collection, and he or she follows the same
-procedure.
-
-
-* Incremental reading
-
-
-An innovative feature of the program SuperMemo is so-called "incremental
-reading". This refers to the ability to quickly and easily make drill items
-from selected portions of text as you read an article (a web page for
-example). See [[http://www.supermemo.com/help/read.htm][the SuperMemo website]] for more on incremental reading.
-
-Much of the infrastructure for incremental reading is already provided by Org
-Mode, with the help of some other emacs packages. You can provide yourself with
-an incremental reading facility by using 'org-capture' alongside a package that
-allows you to browse web pages either in emacs (w3 or [[http://www.emacswiki.org/emacs/emacs-w3m][emacs-w3m]]) or in the
-external browser of your choice ([[http://orgmode.org/worg/org-contrib/org-protocol.php][org-protocol]]).
-
-Another important component of incremental reading is the ability to save your
-exact place in a document, so you can read it /incrementally/ rather than all
-at once. There is a large variety of bookmarking packages for emacs which
-provide advanced bookmarking functionality: see the [[http://www.emacswiki.org/emacs/BookMarks][Emacs Wiki]] for details.
-Bookmarking exact webpage locations in an external browser seems to be a bit
-more difficult. For Firefox, the [[http://www.wired-marker.org/][Wired Marker]] addon works well.
-
-An example of using Org-Drill for incremental reading is given below. First,
-and most importantly, we need to define a couple of =org-capture= templates for
-captured facts.
-
-#+BEGIN_EXAMPLE
-(setq org-capture-templates
-       `(("u"
-         "Task: Read this URL"
-         entry
-         (file+headline "tasks.org" "Articles To Read")
-         ,(concat "* TODO Read article: '%:description'\nURL: %c\n\n")
-         :empty-lines 1
-         :immediate-finish t)
-
-        ("w"
-         "Capture web snippet"
-         entry
-         (file+headline "my-facts.org" "Inbox")
-         ,(concat "* Fact: '%:description'        :"
-                  (format "%s" org-drill-question-tag)
-                  ":\n:PROPERTIES:\n:DATE_ADDED: %u\n:SOURCE_URL: %c\n:END:\n\n%i\n%?\n")
-         :empty-lines 1
-         :immediate-finish t)
-        ;; ...other capture templates...
-    ))
-#+END_EXAMPLE
-
-Using these templates and =org-protocol=, you can set up buttons in your web
-browser to:
-- Create a task telling you to read the URL of the currently viewed webpage
-- Turn a region of selected text on a webpage, into a new fact which is saved
-  to whichever file and heading you nominate in the template. The fact will
-  contain a timestamp, and a hyperlink back to the webpage where you created
-  it.
-
-For example, suppose you are reading the Wikipedia entry on tuberculosis [[http://en.wikipedia.org/wiki/Tuberculosis][here]].
-
-You read the following:
-
-#+BEGIN_QUOTE
-The classic symptoms of tuberculosis are a chronic cough with blood-tinged
-sputum, fever, night sweats, and weight loss. Infection of other organs causes
-a wide range of symptoms. Treatment is difficult and requires long courses of
-multiple antibiotics. Antibiotic resistance is a growing problem in
-(extensively) multi-drug-resistant tuberculosis. Prevention relies on screening
-programs and vaccination, usually with Bacillus Calmette-Guérin vaccine.
-#+END_QUOTE
-
-You decide you want to remember that "Bacillus Calmette-Guérin vaccine" is the
-name of the vaccine against tuberculosis. First, you select the `interesting'
-portion of the text with the mouse:
-
-#+BEGIN_QUOTE
-The classic symptoms of tuberculosis are a chronic cough with blood-tinged
-sputum, fever, night sweats, and weight loss. Infection of other organs causes
-a wide range of symptoms. Treatment is difficult and requires long courses of
-multiple antibiotics. Antibiotic resistance is a growing problem in
-(extensively) multi-drug-resistant tuberculosis.
-@<font style="background-color: yellow;">Prevention relies
-on screening programs and vaccination, usually with Bacillus Calmette-Guérin
-vaccine.@</font>
-#+END_QUOTE
-
-Then you press the button you created when setting up =org-protocol=, which is
-configured to activate the capture template "w: Capture web snippet". The
-selected text will be sent to Emacs, turned into a new fact using the template,
-and filed away for your later attention.
-
-(Note that it might be more efficient to turn the entire paragraph into a drill
-item -- since it contains several important facts -- then split it up into
-multiple items when you edit it later in Emacs.)
-
-Once you have had enough of reading the article, save your place, then go to
-your "fact" file in Emacs. You should see that each piece of text you selected
-has been turned into a drill item. Continuing the above example, you would see
-something like:
-
-#+BEGIN_EXAMPLE
-** Fact: 'Tuberculosis - Wikipedia, the Free Encyclopedia'        :drill:
-
-Prevention relies on screening programs and vaccination, usually with Bacillus
-Calmette-Guérin vaccine.
-#+END_EXAMPLE
-
-You need to edit this fact so it makes sense independent of its context, as
-that is how it will be presented to you in future. The easiest way to turn the
-text into a 'question' is by cloze deletion. All you need to do is surround the
-'hidden' parts of the text with square brackets.
-
-: Prevention of tuberculosis relies on screening programs and vaccination,
-: usually with [Bacillus Calmette-Guérin vaccine].
-
-
-You can of course define browser buttons that use several different "fact"
-templates, each of which might send its fact to a different file or subheading,
-or give it different tags or properties, for example.
-
-