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                       Link Grammar Parser
                          Version 4.7.9

The Link Grammar Parser implements the Sleator/Temperley/Lafferty
theory of natural language parsing. This version of the parser is
an extended, expanded version of the last official CMU release, and
includes many enhancements and fixes created by many different
developers.  This latest version incorporates a substantial part (but
not all) of the BioLG project.

This code is released under the BSD license (which is GPL compatible),
making it freely available for private and commercial use, with
few restrictions.  The terms of the license are given in the
LICENSE file included with this software, and also available at

Please see the web page
for more information.  This version is a continuation of the original
parser posted at

CONTENTS of this directory:
   LICENSE                  The license describing terms of use

   link-grammar/*.c         The program.  (Written in ANSI-C)
   link-grammar/autoit/*    Optional AutoIt language bindings.
   link-grammar/corpus/*.c  Optional corpus statistics database.
   link-grammar/java/*      Optional Java language bindings.
   link-grammar/minisat/*   Optional SAT Solver. (Written in C++)
   link-grammar/sat-solver  Optional SAT Solver. (Written in C++)

   data/en/*                English language dictionaries.
   data/en/4.0.dict         The file containing the dictionary definitions.
   data/en/4.0.knowledge    The post-processing knowledge file.
   data/en/4.0.constituents The constituent knowledge file.
   data/en/4.0.affix        The affix file.
   data/en/4.0.regex        Regex-based morphology guesser.
   data/en/tiny.dict        A small sample dictionary.
   data/en/words/*          A directory full of word lists.
   data/en/4.0*.batch       These files contain sentences (both grammatical
                            and ungrammatical ones) that are handled correctly
                            by this release of the system.  These can be
                            run through the parser with the command
                            "./link-parser < 4.0.*.batch"

   COPYING                  The license for this code and data
   ChangeLog                A compendium of recent changes.
   configure                The GNU configuration script               Developer's configure maintenance tool
   msvc6, msvc9             Microsoft Visual-C project files

UNPACKING and signature verification:
   The system is distributed using the normal tar.gz format; it can be
   extracted using the "tar -zxf link-grammar.tar.gz" command at the
   command line.

   The files have been digitally signed to make sure that there was no
   corruption of the dataset during download, and to help ensure that
   no malicious changes were made to the code internals by third
   parties. The signatures can be checked with the gpg command:

   gpg --verify link-grammar-4.7.8.tar.gz.asc

   which should generate output identical to (except for the date):

   gpg: Signature made Thu 26 Apr 2012 12:45:31 PM CDT using RSA key ID E0C0651C
   gpg: Good signature from "Linas Vepstas (Hexagon Architecture Patches) <>"
   gpg:                 aka "Linas Vepstas (LKML) <>"

   Alternately, the md5 checksums can be verified. These do not provide
   cryptographic security, but they can detect simple corruption. To
   verify the checksums, issue "md5sum -c MD5SUM" at the command line.

CREATING the system:
   To compile the link-grammar shared library and demonstration program,
   at the command line, type:


   To install, change user to "root" and say

        make install

   This will install the library into /usr/local/lib,
   the header files in /usr/local/include/link-grammar, and the
   dictionaries into /usr/local/share/link-grammar.  Running 'ldconfig'
   will rebuild the shared library cache.

   By default, the Makefiles attempt to build the Java bindings.
   The use of the Java bindings is *OPTIONAL*; you do not need these if
   you do not plan to use link-grammar with Java.  You can skip building
   the Java bindings by disabling as follows:

      ./configure --disable-java-bindings

   The /usr/local install target can be over-ridden using the
   standard GNU configure --prefix option, so for example:

      ./configure --prefix=/opt/link-grammar

   By using pkg-config (see below), non-standard install locations
   can be automatically detected.

   Additional config options are printed by

      ./configure --help

   The system has been tested and works well on 32 and 64-bit Linux
   systems, FreeBSD, MacOSX, as well as on many Microsoft Windows
   systems, under various different Windows development environments.
   Specific OS-dependent notes follow.

   Apple MacOSX users will probably need to obtain missing packages
   from MacPorts in order to successfully build on MacOS. See for details.

   See also:

   A MacOS project file can be found in the directory link-grammar.xcode
   It is probably stale and out-of-date; can someone send me a new one?

   Users who intend to use the java bindings with java6 will need to
   make sure to compile link-grammar to produce a 64-bit binary, as
   this is not automatic on OSX. Do this during configure:

      ./configure CFLAGS="-arch x86_64"

   Be sure to set the JDK_HOME environment variable to wherever
   <Headers/jni.h> is.   Set the JAVA_HOME variable to the location of
   the java compiler.

BUILDING on Windows
   There are three different ways in which link-grammr can be compiled
   on Windows. One way is to use Cygwin, which provides a Linux
   compatibility layer for Windows. Unfortunately, the Cygwin system
   is not compatible with Java for Windows.  Another way is use the
   MSVC system. A third way is to use the MinGW system, which uses the
   Gnu toolset to compile windows programs.

   Link-grammar now requires a working version of POSIX-standard regex
   libraries. Since these are not provided by Microsoft, a copy must be
   obtained elsewhere. One possibility is from the gnuwin32 system:
   See also: Perl-Compatible Regular Expressions.

   These different build methods are NOT regularly tested, and some
   linkgrammar versions may have build issues.  If you are an
   experienced Windows developer who knows how to make things work
   in the Microsoft environment, we need your help!

BUILDING on Windows (Cygwin)
   The easiest way to have link-grammar working on MS Windows is to
   use Cygwin, a Linux-like environment for Windows making it possible
   to port software running on POSIX systems to Windows.  Download and
   install Cygwin from

   Unfortunately, the Cygwin system is not compatible with Java, so if
   you need the Java bindings, you must use MSVC or MinGW, below.

BUILDING on Windows (MinGW)
   Another way to build link-grammar is to use the MinGW/MSYS, which
   uses the Gnu toolset to compile Windows programs for Windows. This
   is probably the easiest way to obtain workable Java bindings for
   Windows.  Download and install MinGW, MSYS and MSYS-DTK from

   Then build and install link-grammar with

       make install

   If you used the standard installation paths, the directory /usr/ is
   mapped to C:\msys\1.0, so after 'make install', the libraries and
   executable will be found at C:\msys\1.0\local\bin and the dictionary
   files at C:\msys\1.0\local\share\link-grammar.

   In order to use the Java bindings you'll need to build two extra
   DLLs, by running the following commands from the link-grammar base

       cd link-grammar

       gcc -g -shared -Wall -D_JNI_IMPLEMENTATION_ -Wl,--kill-at \
       .libs/analyze-linkage.o .libs/and.o .libs/api.o \
       .libs/build-disjuncts.o .libs/constituents.o \
       .libs/count.o .libs/disjuncts.o .libs/disjunct-utils.o \
       .libs/error.o .libs/expand.o .libs/extract-links.o \
       .libs/fast-match.o .libs/idiom.o .libs/massage.o \
       .libs/post-process.o .libs/pp_knowledge.o .libs/pp_lexer.o \
       .libs/pp_linkset.o .libs/prefix.o .libs/preparation.o \
       .libs/print-util.o .libs/print.o .libs/prune.o \
       .libs/read-dict.o .libs/read-regex.o .libs/regex-morph.o \
       .libs/resources.o .libs/spellcheck-aspell.o \
       .libs/spellcheck-hun.o .libs/string-set.o .libs/tokenize.o \
       .libs/utilities.o .libs/word-file.o .libs/word-utils.o \
       -o /usr/local/bin/link-grammar.dll

       gcc -g -shared -Wall -D_JNI_IMPLEMENTATION_ -Wl,--kill-at \
       .libs/jni-client.o /usr/local/bin/link-grammar.dll \
       -o /usr/local/bin/link-grammar-java.dll

   This will create link-grammar.dll and link-grammar-java.dll in the
   directory c:\msys\1.0\local\bin . These files, together with
   link-grammar-*.jar, will be used by Java programs.

   Make sure that this directory is in the %PATH setting, as otherwise,
   the DLL's will not be found.

BUILDING on Windows (MSVC)
   Microsoft Visual C/C++ project files can be found in the msvc6 and
   msvc9 directories.

   Please note that the regex package, which includes libraries and
   header files, must be seperately downloaded and installed, as
   described above.  The MSVC project files *MUST* be modified to
   indicate the correct location of the regex libraries.

   The build files make use of two environment variables, GNUREGEX and
   -- GNUREGEX must be pointing to an unzipped gnuwin32-regex
   -- JAVA_HOME must be pointing to a locally installed JDK.

   Those two can be set either as system environment variables (Windows
   users are supposed to know how to do this :) or as MSVC9 user macros.
   But just in case you don't, here's how:

   1) Start > Control Panel > System (remember in Vista or 7 you need
      to switch to "Classic View" or "Large icons" respectively to see
      the System icon).
   2) "Advanced system settings" (or "Advanced" tab under XP)
   3) On all versions you will see a button with the caption
      "Environment Variables", press it.... (ALL REMAINING STEPS
   4) You now see two lists of environment variables... the top one
      says "User variables for <yourusernamehere>" and is localized to
      your user account, the other says "System variables" and applies
      to ALL user accounts on that computer.
   5) Press the "New ..." button corresponding to whether or not you
      want the variables to be valid on ALL accounts or just your own
      (either way the following steps remain the same)
   6) In the "Variable name:" box, enter "GNUREGEX".
   7) In the "Variable value" box, enter the path to your installation
      of GNUREGEX (on my system this is "C:\Program Files (x86)\GnuWin32"
      as I am on Windows 7 Ultimate x64) then press "OK"
   8) Press the same "New ..." button and this time in the "Variable
      name" box enter "JAVA_HOME", and in the "Variable value" box
      enter the path to your Java SDK root folder. (IMPORTANT NOTE: On
      some systems this variable may already be defined automatically
      by the JAVA SDK installation! You should check the variables
      lists before creating a new one to avoid any conflict).
   9) Press "OK" and close all Windows opened during the above steps.

   If you were running MSVC++ or your chosen development environment
   whilst performing the above steps, you should restart it! Once
   restarted you should be able to build the latest version of the

RUNNING the program:
   To run the program issue the Unix command:


   This starts the program.  The program has many user-settable variables
   and options. These can be displayed by entering !var at the link-parser
   prompt.  Entering !help will display some additional commands.

   The dictionaries contain some utf-8 punctuation. These may generate
   errors for users in a non-utf-8 locale, such as the "C" locale.
   The locale can be set, for example, by saying

       export LANG=en_US.UTF-8

   at the shell prompt.

   By default, the parser will use dictionaries at the installed location
   (typically in /usr/local/share). Other locations can be specified on
   the command line; for example:

      link-parser ../path/to-my/modified/data/en

   When accessing dictionaries in non-standard locations, the standard
   filenames are still assumed (i.e. 4.0.dict, 4.0.affix, etc.)

   If you see errors similar to this:

       Warning: The word "encyclop" found near line 252 of en/4.0.dict
       matches the following words:
       This word will be ignored.

   then your UTF-8 locales are either not installed or not configured.
   The shell command `locale -a` should list en_US.utf8 as a locale.
   If not, then you need to `dpkg-reconfigure locales` and/or run
   `update-locale` or possibly `apt-get install locales`, or
   combinations or variants of these, depending on your operating

TESTING the program:
   The program can run in batch mode for testing the system on a large
   number of sentences.  The following command runs the parser on
   a file called 4.0.batch

       ./link-parser < 4.0.batch

   The line "!batch" near the top of 4.0.batch turns on batch mode.  In
   this mode sentences labeled with an initial "*" should be rejected
   and those not starting with a "*" should be accepted.  The current
   batch file does report some errors, as do the files "4.0.biolg.batch"
   and "4.0.fixes.batch".  Work is ongoing to fix these.

   The "4.0.fixes.batch" file contains sentences that have been fixed
   since the original 4.1 release of link-grammar. The "4.0.biolg.batch"
   contains biology/medical-text sentences from the BioLG project.

USING the parser in your own applications:
   There is an API (application program interface) to the parser.  This
   makes it easy to incorporate it into your own applications.  The API
   is documented on the web site.

USING pkg-config:
   To make compiling and linking easier, the current release uses
   the pkg-config system. To determine the location of the link-grammar
   header files, say `pkg-config --cflags link-grammar`  To obtain
   the location of the libraries, say `pkg-config --libs link-grammar`
   Thus, for example, a typical makefile might include the targets:

         cc -O2 -g -Wall -c $< `pkg-config --cflags link-grammar`

      $(EXE): $(OBJS)
         cc -g -o $@ $^ `pkg-config --libs link-grammar`

JAVA bindings:
   This release includes Java bindings.  Their use is optional.

   The bindings will be built automatically if jni.h can be found.
   Some common java JVM distributions (most notably, the ones from Sun)
   place this file in unusual locations, where it cannot be
   automatically found.  To remedy this, make sure that JAVA_HOME is
   set. The configure script looks for jni.h in $JAVA_HOME/Headers
   and in $JAVA_HOME/include; it also examines corresponding locations
   for $JDK_HOME.  If jni.h still cannot be found, specify the location
   with the CPPFLAGS variable: so, for example,

      export CPPFLAGS="-I/opt/jdk1.5/include/:/opt/jdk1.5/include/linux"
      export CPPFLAGS="-I/c/java/jdk1.6.0/include/ -I/c/java/jdk1.6.0/include/win32/"

   Please note that the use of /opt is non-standard, and most system
   tools will fail to find packages installed there.

   The building of the Java bindings can be disabled by configuring as

      ./configure --disable-java-bindings

Spell Checking:
   The parser will run a spell-checker at an early stage, if it
   encounters a word that it does not know, and cannot guess, based on
   morphology.  The configure script looks for the aspell or hunspell
   spell-checkers; if the aspell devel environment is found, then
   aspell is used, else hunspell is used.

   Spell checking may be disabled at runtime, in the link-parser client
   with the !spell flag.  Enter !help for more details.

Corpus Statistics:
   The parser now contains some experimental code for using corpus
   statistics to provide a parse ranking, and to assign WordNet word
   senses to word, based on their grammatical usage.  An overview of
   the idea is given on the OpenCog blog, here:

   It is planned that the Corpus statistics database will be used to
   guide the SAT solver.

   To enable the corpus statistics, specify

      ./configure --enable-corpus-stats

   prior to compiling.  The database itself can be downloaded from

   The data is contained in an sqlite3 database file,


   Unzip this file (using bunzip2) rename it to "disjuncts.db", and
   place it in the subdirectory "sql", in the same directory that
   contains the "en" directory. For default unix installations, the
   final location would be


   where, by comparison, the usual dictionary would be at


   After this is installed, parse ranking scores should be printed
   automatically, as floating-point numbers: for example:

      Unique linkage, cost vector = (CORP=4.4257 UNUSED=0 DIS=1 AND=0 LEN=5)

   Lower numbers are better.  The scores can be interpreted as -log_2
   of a certain probability, so the lower the number, the higher the

   The display of disjunct scores can be enabled with the !disjuncts
   flag, and senses with the !senses flag, at the link-parser prompt.
   Entering !var and !help will show all flags.  Multiple parses are
   sorted and displayed in order from lowest to highest cost; the sort
   of can be set by saying !cost=1 for the traditional sort, and
   !cost=2 for corpus-based cost.  Output similar to the below should
   be printed:

      linkparser> !disjunct
      Showing of disjunct used turned on.
      linkparser> !cost=2
      cost set to 2
      linkparser> !sense
      Showing of word senses turned on.
      linkparser> this is a test
      Found 1 linkage (1 had no P.P. violations)
      Unique linkage, cost vector = (CORP=4.4257 UNUSED=0 DIS=1 AND=0 LEN=5)

         +-Ss*b+  +-Ds-+
         |     |  |    |
      this.p is.v a test.n

      2 is.v dj=Ss*b- Ost+  sense=be%2:42:02:: score=2.351568
      2 is.v dj=Ss*b- Ost+  sense=be%2:42:05:: score=2.143989
      2 is.v dj=Ss*b- Ost+  sense=be%2:42:03:: score=1.699292
      4 test.n dj=Ost- Ds-  sense=test%1:04:00:: score=0.000000
                     this.p      0.0  0.695 Wd- Ss*b+
                       is.v      0.0  7.355 Ss*b- Ost+
                          a      0.0  0.502 Ds+
                     test.n      1.0  9.151 Ost- Ds-

   Note that the sense labels are not terribly accurate; the verb "to be"
   is particularly hard to tag correctly.

   The link-grammar library should be thread-safe as of version 4.3.6,
   although this has not yet been tested very well, and may possibly be
   buggy.  To enable the thread-safe version of the library, run
   ./configure like so:

      ./configure --enable-pthreads

   The resulting library is thread-safe for all parsing and general
   use, with the exception of the startup/initialization code.  That
   is, the initial open/creation of the parser dictionary should be
   done only once, from a single thread, and parsing should not be
   started until after the dictionary has been read and initialized.

   Parse options can be set on a per-thread basis, with the exception
   of verbosity, which is treated as a global.

   The library uses pthreads for the few spots where it needs to manage
   per-thread storage. It is possibly quite reasonable to abolish these
   few spots, thus making the library not only thread-safe, but thread
   agnostic (i.e. not require pthreads).

   The following exceptions and special notes apply:

      utilities.c -- has global "verbosity". Uses pthreads for tracking
                     memory usage. The memory usage code is just about
                     obsolete, and could probably be discarded.
      jni-client.c - uses per-thread struct, but needs to be attached
                     to JNIEnv somehow.
      malloc-dbg.c - not thread safe, not normally used;
                     only for debugging.
      prefix.c     - not thread-safe, but doesn't need to be; used only
                     during initialization, and only if binreloc turned
      pp_lexer.c  -- autogened code, original lex sources lost.
                     This is only used when reading dictionaries,
                     during initialization, and so doesn't need
                     to be thread safe.

BioLG merger:
   Much of the BioLG project changes have been merged into this version.
   The current version of link-grammar has superior parse coverage to BioLG
   on all texts, including biomedical texts.  The original BioLG test suite
   can be found in data/en/4.0.biolg.batch.

   The following changes in BioLG have NOT been merged:
   -- Part of speech hinting. The BioLG code can accept part-of-speech
      hints for unknown words.
   -- XML I/O. The BioLG code can output parsed text in a certain
      idiosyncratic XML format.
   -- "term support". Link-grammar does support "entity placeholders",
      which provides an equivalant function.
   -- The link type CH. This was a large, intrusive, incompatible change
      to the dictionary, and it is not strictly required -- there is a
      better, alternative way of handling adj-noun-adj-noun chains commonly
      seen in biomedical text, and this has been implemented.

   All other BioLG changes, and in particular, extensive dictionary fixes,
   as well as regex morphology handling, have been incorporated.

Medical Terms Merger
Many, but not all, of the "medical terms" from Peter Szolovits have been
merged into version 4.3.1 (January 2008) of link-grammar. The origiginal
project page was at:

The following "extra" files were either merged directly, renamed, or
skipped (omitted):

/extra.1: -- merged
/extra.2: -- skip, too big
/extra.3: -- skip, too big
/extra.4: -- /en/words/words-medical.v.4.2:
/extra.5: -- /en/words/words-medical.v.4.1:
/extra.6: -- /en/words/words-medical.adj.2:
/extra.7: -- /en/words/words-medical.n.p
/extra.8: -- skip, too big
/extra.9: -- skip, random names
/extra.10: -- /en/words/words-medical.adv.1:
/extra.11: -- /en/words/words-medical.v.4.5:
/extra.12: -- skip, too big
/extra.13: -- /en/words/words-medical.v.4.3:
/extra.14: -- /en/words/words-medical.prep.1
/extra.15: -- /en/words/words-medical.adj.3:
/extra.16: -- /en/words/words-medical.v.2.1:
/extra.17: -- skip, too big

The make use of the "skipped" files, download the original extension,
gut the contents of "extra.dict" except for the parts refering to the
skipped files above, and then append to 4.0.dict (as per original

Fat Links:
   As of version 4.7.0, parsing using "fat links" has been disabled
   by default, and is now deprecated.  The function is still there,
   and can be turned on by specifying the !use-fat=1 command, or by
   calling parse_options_use_fat_links(TRUE) from programs.

   Older versions of the link-grammar parser used "fat links" to
   support conjunctions (and, or, but, ...). However, this lead
   to a number of complications, including poor performance due to
   a combinatorial explosion of linkage possibilities, as well as
   an excessively complex parse algorithm.  As a result, the function
   is now deprecated, and will be removed in version 4.8.

SAT solver:
   The current parser uses an algorithm that runs in O(N^3) time, for
   a sentence containing N words. This algorithm is more or less a
   chart parser, a variant of the Earley parser.

   The SAT solver aims to replace the chart parser with an algorithm
   based on Boolean Satisfiability Theory; specifically using the
   MiniSAT solver. The SAT solver has a bit more overhead for shorter
   sentences, but is significantly faster for long sentences.  To work
   properly, it needs to be attached to a parse ranking system.  This
   work is incomplete, although the prototype works.  it is not yet
   well-integrated with the system, and needs cleanup.

   The SAT solver is enabled by specifying

      ./configure --enable-sat-solver

   prior to compiling.

   The original authors would be pleased to discuss alternate
   licensing terms and/or additional research work, with those
   wanting to use this system under a paid contract.

   Academic products or papers should reference the various
   link grammar publications.

   If you have any questions, or find any bugs, please feel free
   to send a note to the mailing list:

   Although all messages should go to the mailing list, the current
   maintainers can be contacted at:

     Linas Vepstas - <>
     Dom Lachowicz - <>

   The original authors of the Link Grammar parser are:

     Daniel Sleator          
     Computer Science Department       412-268-7563
     Carnegie Mellon University
     Pittsburgh, PA 15213

     Davy Temperley          
     Eastman School of Music           716-274-1557
     26 Gibbs St.            
     Rochester, NY 14604

     John Lafferty           
     Computer Science Department       412-268-6791
     Carnegie Mellon University
     Pittsburgh, PA 15213

TODO -- Working Notes:
  Some working notes.

  Easy to fix: provide a more uniform API to the constituent tree.
  i.e provide word index.   Also .. provide a clear word API,
  showing word extent, suffix, etc.

  Head words:
    The head-word of a sentences is usually the verb; head-words should
    attach to walls, and not nouns, the way the current link-grammar
    does it.  Why?  Because this allows for a more correct handling of
    many clause openers, such as "the blah being blah, ..." e.g
      * Cutting was difficult, the knife being dull.
    In the above, "being" can only be used in this way, in such compound
    sentences.  The head-word is already correctly linked with C for
    subordinate clauses.  "I told him I was angry", with "told" being the

  Zero/phantom words:  Expressions such as "Looks good" have an implicit
    "it" (also called a zero-it or phantom-it) in them; that is, the
    sentence should really parse as "(it) looks good".  The dictionary
    could be simplified by admitting such phantom words explicitly,
    rather than modifying the grammar rules to allow such constructions.
    Other examples, with the phantom word in parenthesis, include:
    * I ate all (of) the cookies.
    * I taught him (how) to swim.

  Bad grammar: When a sentence fails to parse, look for:
    * confused words: its/it's, there/their, to/too, your/you're ...
    * missing apostrophes in possessives: "the peoples desires"
    * determiner agreement errors: "a books"
    * aux verb agreement errors: "to be hooks up"

  A/An determiners before consonants/vowels:  fix this. Just adding
    linkages in a naive way is not enough, because there's no restriction
    on neighboring words: "I ate an apple." vs. "I ate a green apple".
    Should this be done in post-processing? or in pre-processing?
    (maybe easier to post-process, but more biologically natural as
    a pre-process step.)

  Poor linkage choices:
    Compare "she will be happier than before" to "she will be more happy
    than before." Current parser makes "happy" the head word, and "more"
    a modifier w/EA link.  I beleive the correct solution would be to
    make "more" the head (link it as a comparative), and make "happy"
    the dependent.  This would harmonize rules for comparatives... and
    would eliminate/simplify rules for less,more.

  Repulsive parses: Sometimes, the existance of one parse should suggest
    that another parse must surely be wrong: if one parse is possible,
    then the other parses must surely be unlikely. For example: the
    conjunction and.j-g allows the "The Great Southern and Western
    Railroad" to be parsed as the single name of an entity. However,
    it also provides a pattern match for "John and Mike" as a single
    entity, which is almost certainly wrong. But "John and Mike" has
    an alternative parse, as a conventional-and -- a list of two people,
    and so the existance of this alternaive (and correct) parse suggests
    that perhaps the entity-and is really very much the wrong parse.
    That is, the mere possibility of certain parses should strongly
    disfavour other possible parses. (Exception: Ben & Jerry's ice
    cream; however, in this case, we could recognize Ben & Jerry as the
    name of a proper brand; but this is outside of the "normal"
    dictionary (?) (but maybe should be in the dictionary!))

    More examples: "high water" can have A joining high.a and AN joining
    high.n; these two should either be collapsed into one, or one should
    be eliminated.

  Wordnet hinting:
    Use wordnet to reduce the number for parses for sentences containing
    compound verb phrases, such as "give up", "give off", etc.

  incremental parsing: to avoid a combinatorial explosion of parses,
    it would be nice to have an incremental parsing, phrase by phrase,
    using a Viterbi-like algorithm to obtain the parse. Thus, for example,
    the parse of the last half of a long, run-on sentence should not be
    sensitive to the parse of the beginning of the sentence.

    Doing so would help with combinatorial explosion. So, for example,
    if the first half of a sentence has 4 plausible parses, and the
    last half has 4 more, then link-grammar reports 16 parses total.
    It would be much, much more useful to instead be given the
    factored results: i.e. the four plausible parses for the
    first half, and the four plausible parses for the last half.
    The lower combinatoric stress would ease the burden on
    downstream users of link-grammar.

    (This somewhat resembles the application of construction grammar
    ideas to the link-grammar dictionary).

     Caution: watch out for garden-path sentences:
     The horse raced past the barn fell.
     The old man the boat.
     The cotton clothing is made of grows in Mississippi.
     The current parser parses these perfectly; a viterbi parsr could
     trip on these.

   Other benefits of a Viterbi decoder:
   * Less sensitive to sentence boundaries: this would allow longer,
     run-on sentences to be parsed far more quickly.
   * Could do better with slang, hip-speak.
   * Would enable co-reference resultion across sentences (resolve
     pronouns, etc.)
   * Would allow richer state to be passed up to higher layers:
     specifically, alternate parses for fractions of a sentence,
     alternate refernce resolutions.
   * Would allow plug-in architecture, so that plugins, employing
     some alternate, higher-level logic, could disambiguate (e.g.
     by making use of semantic content).
   * Eliminate many of the hard-coded array sizes in the code.
   * Fixes the word-count problem during spell-guessing. So, for
     example, if the mis-spelled word "dont" shows up in the input, it
     could be issued as one word ("done") or two ("do n't") and the
     current suffix-stripping/word-issueing algo cannot deal with this
     correctly. By contrast, this should not be an issue for the 
     Viterbi algo, as it could explore both states at once.
   One may argue that Viterbi is a more natural, biological way of
   working with sequences.  Some experimental, psychological support
   for this can be found here:
   per Morten Christiansen, Cornell professor of psychology.

      Consider the sentence "Thieves rob bank" -- a typical newspaper
      headline. LG currently fails to parse this, because the determiner
      is missing ("bank" is a count noun, not a mass noun, and thus
      requires a determiner. By contrast, "thieves rob water" parses
      just fine.) A fix for this would be to replace mandatory
      determiner links by (D- or {[[()]] & headline-flag}) which allows
      the D link to be ommitted if the headline-flag bit is set.
      Here, "headline-flag" could be a new link-type, but one that is
      not subject to planarity constraints.

      Note that this is easier said than done: if one simply adds a
      high-cost null link, and no headline-flag, then all sorts of
      ungrammatical sentences parse, with strange parses; while some
      grammatical sentences, which should parse, but currently don't,
      become parseable, but with crazy results.

   Hand-refining verb patterns:
      A good reference for refining verb usage patterns is:
      online at

   Quotations: Currently, tokenize.c ignores all ASCII double-quotes
      (grep for "quote_found" in the source). However, it does not do this
      for the various "curly" UTF8 quotes, such as ‘these’ and “these”.
      This results is some ugly parsing for sentences containing such
      quotes. (Note that these are in 4.0.affix).

   no-links-cross: Link Grammar uses a constraint that all linkages must
      be planar graphs. While this is mostly correct, it would probably
      be more correct to use "landmark transitivity" as articulated by
      Hudson in the Word Grammar theory.  This might allow the elimination
      of most or all post-processing rules.

      This is done by making each link directional: one end of the link
      is the parent. (e.g. noun, noun-modifier: the noun is the parent).
      Parents are landmarks for children.  Transitivity is applied to
      parent-child relationships. Specifically, the no-links-cross
      rule is replaced by two landmark transitivity rules:

      -- If B is a landmark for C, then  A is also a type-L landmark for C
      -- If A is a landmark for C, then B is also a landmark for C

      where type-L means either a right-going or left-going link.

     See for details.
     See also:

  "to be fishing": Link grammar offers four parses of "I was fishing for
     evidence", two of which are given low scores, and two are given
     high scores. Of the two with high scores, one parse is clearly bad.
     Its links "to be fishing.noun" as opposed to the correct
     "to be fishing.gerund". That is, I can be happy, healthy and wise,
     but I certainly cannot be fishing.noun.  This is perhaps not
     just a bug in the structure of the dictionary, but is perhaps
     deeper: link-grammar has little or no concept of lexical units
     (i.e. collocations, idioms, institutional phrases), which thus
     allows parses with bad word-senses to sneak in.

     The goal is to introduce more knowledge of lexical units into LG.

     Different word senses can have different grammar rules (and thus,
     the links employed reveal the sense of the word): for example:
     "I tend to agree" vs. "I tend to the sheep" -- these employ two
     different meanings for the verb "tend", and the grammatical
     constructions allowed for one meaning are not the same as those
     allowed for the other. Yet, the link rules for "tend.v" have
     to accommodate both senses, thus making the rules rather complex.
     Worse, it potentially allows for non-sense constructions.
     If, instead, we allowed the dictionary to contain different
     rules for "tend.meaning1" and "tend.meaning2", the rules would
     simplify (at the cost of inflating the size of the dictionary).

     Another example: "I fear so" -- the word "so" is only allowed
     with some, but not all, lexical senses of "fear". So e.g.
     "I fear so" is in the same semantic class as "I think so" or
     "I hope so", although other meanings of these verbs are
     otherwise quite different.

     [Sin2004] "New evidence, new priorities, new attitudes" in J.
     Sinclair, (ed) (2004) How to use corpora in language teaching,
     Amsterdam: John Benjamins

     See also: Pattern Grammar: A Corpus-Driven Approach to the Lexical
     Grammar of English Susan Hunston and Gill Francis (University of
     Birmingham) Amsterdam: John Benjamins (Studies in corpus linguistics,
     edited by Elena Tognini-Bonelli, volume 4), 2000

  "holes" in collocations (aka "set phrases" of "phrasemes"):
     The link-grammar provides several mechanisms to support
     circumpositions or even more complicated multi-word structures.
     One mechanism is by ordinary links; see the V, XJ and RJ links.
     The other mechanism is by means of post-processing rules.
     However, rules for many common forms have not yet been written.
     The general problem is of supporting structures that have "holes"
     in the middle. For example, the adposition:

        ... from [xxx] on.

            "He never said another word from then on."
            "I promise to be quiet from now on."
            "Keep going straight from that point on."
            "We went straight from here on."
            ... from there on.
            "We went straight, from the house on to the woods."
            "We drove straight, from the hill onwards."

    Note that multiple words can fit in the slot [xxx].
    Note the tangling of another prepositional phrase:
    "... from [xxx] on to [yyy]"

    More complicated collocations with holes include
    "First.. next..."
    "If ... then ..."

    'Then' is optional, for example:
    "If it is raining, stay inside!"

    "if ... only ..." "If there were only more like you!"
    "... not only, ... but also ..."

    "Either ... or ..."
    "Both ... and  ..."  "Both June and Tom are coming"
    "ought ... if ..." "That ought to be the case, if John is not lying"

    "Someone ... who ..."
    "Someone is outside who wants to see you"

    The above are not currently supported. An example that is supported
    is the "non-referential it", e.g.
    "It ... that ..."
    "It seemed likely that John would go"

    The above is supported by means of special disjuncts for 'it' and
    'that', which must occur in the same post-processing domain.

    See also:

    "...from X and from Y"
    "By X, and by Y, ..."
    Here, X and Y might be rather long phrases, containing other
    prepositions. In this case, the usual link-grammar linkage rules
    will typically conjoin "and from Y" to some preposition in X,
    instead of the correct link to "from X". Although adding a cost to
    keep the lengths of X and Y approximately equal can help, it would
    be even better to recognize the "...from ... and from..." pattern.

    One possible way to implement this would be to give preference to
    and-links that have the same word on either end.

    More idiomatic than the above examples:
    "...the chip on X's shoulder"
    "to do X a favour"
    "to give X a look"

    The above are all examples of "set phrases" or "phrasemes", and are
    most commonly discussed in the context of MTT or Meaning-Text Theory
    of Igor Mel'cuk et al (search for "MTT Lexical Function" for more
    info). Mel'cuk treats set phrases as lexemes, and, for parsing, this
    is not directly relevent. However, insofar as phrasemes have a high
    mutual information content, they can dominate the syntactic
    structure of a sentence.

    MTT suggests that perhaps the correct way to understand the contents
    of the post-processing rules is as an implementation of 'lexical
    functions' projected onto syntax.  That is, the post-processing
    rules allow only certain syntactical constructions, and these are
    the kinds of constructions one typically sees in certain kinds
    of lexical functions.

    Alternately, link-grammar suffers from a combinatoric explosion
    of possible parses of a given sentence. It would seem that lexical
    functions could be used to rule out many of these parses.  On the
    other hand, the results are likely to be similar to that of
    statistical pare ranking (which presumably captures such
    quasi-idiomatic collocations at least weakly).

    Ref. I. Mel'cuk: "Collocations and Lexical Functions", in ''Phraseology:
    theory, analysis, and applications'' Ed. Anthony Paul Cowie (1998)
    Oxford University Press pp. 23-54.

  Conjunction cleanup: parser has some special-case code for dealing
    with conjunctions. Unfortunately, some of this code has hard-wired
    English words in them. These are in construct_either() and in
    set_is_conjunction().  Actually, all of massage.c Grep for XXX FIXME.
    All of this can now be deleted for version 5.0.

   Assorted minor cleanup:
      -- Should provide a query that returns compile-time consts,
         e.g. the max number of characters in a word, or max words
         in a sentence
      -- Should remove compile-time constants, e.g. max words, max
         length etc.
      -- Split out parse-options, so that the command-line client
         options are not in the parse-options struct.

  Version 5.0 TODO list:
    Version 5.0 will completely eliminate both fat links, and vestiges thereof.
    In particular, this will get rid of sublinkages, and all uses thereof.
    -- in api.c, change many linkage_*() funs to linkage_get_*() funs
    -- get rid of sublinkages.
    -- Change Sentence, Linkage etc. to not be pointers.