# UMLS::Similarity::lesk.pm
#
# Module implementing the semantic relatedness measure described
# by Banerjee and Pedersen(2002)
#
# Copyright (c) 2009-2011,
#
# Bridget T McInnes, University of Minnesota, Twin Cities
# bthomson at umn.edu
#
# Siddharth Patwardhan, University of Utah, Salt Lake City
# sidd at cs.utah.edu
#
# Serguei Pakhomov, University of Minnesota, Twin Cities
# pakh002 at umn.edu
#
# Ted Pedersen, University of Minnesota, Duluth
# tpederse at d.umn.edu
#
# Ying Liu, University of Minnesota, Twin Cities
# liux0935 at umn.edu
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 2
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to
#
# The Free Software Foundation, Inc.,
# 59 Temple Place - Suite 330,
# Boston, MA 02111-1307, USA.
package UMLS::Similarity::lesk;
use strict;
use warnings;
use UMLS::Similarity;
use Text::OverlapFinder;
use Lingua::Stem::En;
use UMLS::Similarity::ErrorHandler;
use vars qw($VERSION);
$VERSION = '0.07';
my $compoundfile = "";
my $debugfile = "";
my $stoplist = "";
my $stopregex = "";
my $finder = "";
my $dictfile = "";
my $doubledef = "";
my $defraw_option = 0;
my $stem = "";
my $stemmed_words = "";
my $score = 0;
my %dictionary = ();
my %ddef = ();
my %complist = ();
local(*DEBUG);
sub new
{
my $className = shift;
return undef if(ref $className);
my $interface = shift;
my $params = shift;
$params = {} if(!defined $params);
$stoplist = $params->{'stoplist'};
$debugfile = $params->{'debugfile'};
$dictfile = $params->{'dictfile'};
$doubledef = $params->{'doubledef'};
$stem = $params->{'stem'};
$compoundfile = $params->{'compoundfile'};
my $defraw = $params->{'defraw'};
my $self = {};
# Bless the object.
bless($self, $className);
# set the finder handler
$finder = Text::OverlapFinder->new;
# The backend interface object.
$self->{'interface'} = $interface;
# check the configuration file if defined
my $errorhandler = UMLS::Similarity::ErrorHandler->new("lesk", $interface);
if(!$errorhandler) {
print STDERR "The UMLS::Similarity::ErrorHandler did not load properly\n";
exit;
}
if(defined $defraw) {
$defraw_option = 1;
}
if(defined $debugfile) {
if(-e $debugfile) {
print "Debug file $debugfile already exists! Overwrite (Y/N)? ";
my $reply = <STDIN>;
chomp $reply;
$reply = uc $reply;
exit 0 if ($reply ne "Y");
}
open(DEBUG, ">$debugfile")
|| die "Could not open debug file: $debugfile\n";
$debugfile = 1;
}
# Check for stoplist
if(defined $stoplist) {
open(STP, $stoplist) || die "Could not open stoplist: $stoplist\n";
$stopregex = "(";
while(<STP>) {
chomp;
if ($_ ne ""){
$_=~s/\///g;
$stopregex .= "$_|";
}
}
chop $stopregex; $stopregex .= ")";
close STP;
}
# read in the doubledef for --doubledef option
if (defined $doubledef) {
open(DDEF, "<$doubledef")
or die("Error: cannot open doubledef file ($doubledef).\n");
while(<DDEF>) {
chomp;
if($_=~/^\s*$/) { next; }
my @defs = split (":", $_);
my $concept = $defs[0];
$concept =~ s/^\s+//;
$concept =~ s/\s+$//;
my $definition = $defs[1];
$ddef{$concept} = $definition;
}
close DDEF;
}
if(defined $dictfile) {
open(DICT, "$dictfile")
|| die("Error: cannot open dictionary file ($dictfile)\n");
while(<DICT>) {
chomp;
if($_=~/^\s*$/) { next; }
my @defs = split(":", $_);
my $concept = $defs[0];
$concept =~ s/^\s+//;
$concept =~ s/\s+$//;
my $definition = $defs[1];
$dictionary{$concept} = $definition;
}
close DICT;
}
if(defined $compoundfile) {
#replace the compound words in the definition
open(LST, "$compoundfile") or die ("Error: cannot open file $compoundfile for input.\n");
# read the compound txt and put them in the hash array.
while (my $line = <LST>)
{
chomp($line);
my $lower_case = lc($line);
my @string = split('_', $lower_case);
my $head = shift(@string);
my $rest = join (' ', @string);
push (@{$complist{$head}}, $rest);
}
close LST;
# sort the compound txt
foreach my $h (sort (keys (%complist)) )
{
my @sort_list = sort(@{$complist{$h}});
for my $i (0..$#sort_list)
{
$complist{$h}[$i] = $sort_list[$i];
}
}
}
return $self;
}
sub getRelatedness
{
my $self = shift;
return undef if(!defined $self || !ref $self);
my $concept1 = shift;
my $concept2 = shift;
if(defined $debugfile) {
print DEBUG "$concept1<>$concept2\n";
}
# set up the interface
my $interface = $self->{'interface'};
my $def1 = "";
my $def2 = "";
if(!defined $dictfile) {
if($concept1 =~ /C[0-9]+/)
{
my $defs1 = $interface->getExtendedDefinition($concept1);
if(defined $debugfile) {
print DEBUG "DEFINITIONS FOR CONCEPT 1: $concept1 \n";
}
my $i = 1;
foreach my $def (@{$defs1}) {
if(defined $debugfile) {
print DEBUG "$i. $def\n";
$i++;
}
$def=~/(C[0-9]+) ([A-Za-z]+) ([A-Za-z0-9]+) ([A-Za-z0-9\.]+) \s*\:\s*(.*?)$/;
$def1 .= $5 . " " . "definitionstop" . " ";
}
#if the definition is empty, return -1;
if($def1 eq "")
{
return -1;
}
}
if($concept2 =~ /C[0-9]+/)
{
my $defs2 = $interface->getExtendedDefinition($concept2);
if(defined $debugfile) {
print DEBUG "DEFINITIONS FOR CONCEPT 2: $concept2 \n";
}
my $i = 1;
foreach my $def (@{$defs2}) {
if(defined $debugfile) {
print DEBUG "$i. $def\n";
$i++;
}
$def=~/(C[0-9]+) ([A-Za-z]+) ([A-Za-z0-9]+) ([A-Za-z0-9\.]+) \s*\:\s*(.*?)$/;
$def2 .= $5 . " " . "definitionstop" . " ";
}
#if the definition is empty, return -1;
if($def2 eq "")
{
return -1;
}
}
} # end of WITHOUT --dictfile option
if(defined $dictfile){
my $defs1;
my $defs2;
my $term1;
my $term2;
my $term1_def = "";
my $term2_def = "";
my @dictfile_term1;
my @dictfile_term2;
if($concept1 =~ /^(C[0-9]+)(\#)(.*?)$/)
{
my $cui1 = $1;
$term1 = $3;
$defs1 = $interface->getExtendedDefinition($cui1);
$term1_def = $dictionary{$term1} if (defined $dictionary{$term1});
# check the cui's associated term's def in dictfile
#@dictfile_term1 = $interface->getTermList($cui1);
#foreach my $t (@dictfile_term1)
#{
#if(defined ($dictionary{$t}))
#{
#my $term1_def = $dictionary{$t};
#$def1 .= "$term1_def" . " ";
#}
#}
}
else
{
if (defined $dictionary{$concept1}) {
$term1_def = $dictionary{$concept1};
}
else{
if (defined $debugfile) {
print DEBUG "$concept1: not defined\n"; }
return -1; }
}
if($concept2 =~ /^(C[0-9]+)(\#)(.*?)$/)
{
my $cui2 = $1;
$term2 = $3;
$defs2 = $interface->getExtendedDefinition($cui2);
$term2_def = $dictionary{$term2} if (defined $dictionary{$term2});
# check the cui's associated term's def in dictfile
#@dictfile_term2 = $interface->getTermList($cui2);
#foreach my $t (@dictfile_term2)
#{
#if(defined ($dictionary{$t}))
#{
#my $term2_def = $dictionary{$t};
#$def2 .= "$term2_def" . " ";
#}
#}
}
else
{
if (defined $dictionary{$concept2}) {
$term2_def = $dictionary{$concept2};
}
else{
if (defined $debugfile) {
print DEBUG "$concept2: not defined\n"; }
return -1; }
}
# if debug setting is on print out definition one information
if(defined $debugfile) { print DEBUG "DEFINITIONS FOR CONCEPT 1: $concept1 \n"; }
# set up the definition string - note the format is:
# CUI REL CUI SAB : <definition>
my $i = 1;
foreach my $def (@{$defs1}) {
if(defined $debugfile) {
print DEBUG "$i. $def\n";
$i++;
}
$def=~/(C[0-9]+) ([A-Za-z]+) ([A-Za-z0-9]+) ([A-Za-z0-9\.]+) \s*\:\s*(.*?)$/;
$def1 .= $5 . " " . "definitionstop" . " ";
}
if(defined $term1_def)
{
if(defined $debugfile)
{
print DEBUG "$i. $term1_def\n";
}
$def1 .= $term1_def;
}
#if the definition is empty, return -1;
if($def1 eq "")
{
return -1;
}
# if debug setting is on print out definition two information
if(defined $debugfile) { print DEBUG "DEFINITIONS FOR CONCEPT 2: $concept2 \n"; }
# set up the definition string - note the format is:
# CUI REL CUI SAB : <definition>
my $j = 1;
foreach my $def (@{$defs2}) {
if(defined $debugfile) {
print DEBUG "$j. $def\n";
$j++;
}
$def=~/(C[0-9]+) ([A-Za-z]+) ([A-Za-z0-9]+) ([A-Za-z0-9\.]+) \s*\:\s*(.*?)$/;
$def2 .= $5 . " " . "definitionstop" . " ";
}
if(defined $term2_def)
{
if(defined $debugfile)
{
print DEBUG "$j. $term2_def\n";
}
$def2 .= $term2_def;
}
#if the definition is empty, return -1;
if($def2 eq "")
{
return -1;
}
} # end of WITH --dictfile option
if (defined $doubledef)
{
my @def1_array = split(/\s/, $def1);
my @def2_array = split(/\s/, $def2);
my %unique = (); # for every word, only check its definition once
foreach my $w (@def1_array)
{
$unique{$w}++;
if ((defined $ddef{$w}) and ($unique{$w}==1))
{
my $def = $ddef{$w};
if (defined $debugfile)
{
print DEBUG "ddef1 $w: $def\n";
}
$def1 .= "$def" . " ";
}
}
%unique = ();
foreach my $w (@def2_array)
{
$unique{$w}++;
if ((defined $ddef{$w}) and ($unique{$w}==1))
{
my $def = $ddef{$w};
if (defined $debugfile)
{
print DEBUG "ddef2 $w: $def\n";
}
$def2 .= "$def" . " ";
}
}
if (defined $debugfile)
{
print DEBUG "after --doubledef processing\n";
print DEBUG "concept 1: $def1\n";
print DEBUG "concept 2: $def2\n";
}
} #end of defined --doubledef option
# if the --defraw option is not set clean up the defintions
if($defraw_option == 0)
{
$def1 = lc($def1); $def2 = lc($def2);
# remove punctuation doesn't contain '<' and '>'
$def1=~s/[\.\,\?\/\'\"\;\:\[\]\{\}\!\@\#\$\%\^\&\*\(\)\-\_\+\-\=]//g;
$def2=~s/[\.\,\?\/\'\"\;\:\[\]\{\}\!\@\#\$\%\^\&\*\(\)\-\_\+\-\=]//g;
if (defined $debugfile)
{
print DEBUG "after --defraw processing\n";
print DEBUG "concept 1: $def1\n";
print DEBUG "concept 2: $def2\n";
}
}
if(defined $compoundfile)
{
$def1 = findCompoundWord($def1, \%complist);
$def2 = findCompoundWord($def2, \%complist);
if (defined $debugfile)
{
print DEBUG "after --compoundfile processing\n";
print DEBUG "concept 1: $def1\n";
print DEBUG "concept 2: $def2\n";
}
}
# remove stop words
if (defined $stoplist)
{
my @d1 = split(/\s/, $def1);
my @d2 = split(/\s/, $def2);
my $new_def1 = "";
my $new_def2 = "";
foreach my $check (@d1)
{
if(!($check =~ /$stopregex/))
{
$new_def1 .= "$check ";
}
}
foreach my $check (@d2)
{
if(!($check =~ /$stopregex/))
{
$new_def2 .= "$check ";
}
}
$def1 = $new_def1;
$def2 = $new_def2;
if (defined $debugfile)
{
print DEBUG "after --stoplist processing\n";
print DEBUG "concept 1: $def1\n";
print DEBUG "concept 2: $def2\n";
}
}
if (defined $stem)
{
my @def1_words = split(/\s/, $def1);
my @def2_words = split(/\s/, $def2);
my $stemmed_words1 = Lingua::Stem::En::stem({ -words => \@def1_words, -locale => 'en'});
my $stemmed_words2 = Lingua::Stem::En::stem({ -words => \@def2_words, -locale => 'en'});
$def1 = join(" ", @{$stemmed_words1});
$def2 = join(" ", @{$stemmed_words2});
if (defined $debugfile)
{
print DEBUG "after --stem processing\n";
print DEBUG "concept 1: $def1\n";
print DEBUG "concept 2: $def2\n";
}
}
# find the overlap
my $overlaps = "";
my $len1 = 0;
my $len2 = 0;
($overlaps, $len1, $len2) = $finder->getOverlaps($def1, $def2);
# calculate lesk on the overlaps which doesn't cross defs
if (defined $debugfile)
{
print DEBUG "Overlap string and their value\n";
}
my $score = 0;
foreach my $overlap (keys %{$overlaps})
{
#my $length = length ($overlap);
#print "overlap: $length\n";
my $length = 0;
my $num = 0;
my $value = 0;
if ($overlap =~ /definitionstop/)
{
my @array = split/\s+/, $overlap;
my $array_length = $#array + 1;
my $i = 0;
my $overlap_string = "";
foreach my $s (@array)
{
$i++;
if ($s !~ /definitionstop/) # count the length of the overlap
{
$length++;
$overlap_string .= "$s ";
}
if (($s =~ /definitionstop/) and ($length<$array_length) and ($length > 0)) # definitionstop in the middle of the overlap
{
$num = $overlaps->{$overlap};
$value = $num * ($length**2);
$score += $value;
$length = 0;
if (defined $debugfile)
{
print DEBUG "$overlap_string $value\n";
}
$overlap_string = "";
}
if (($s =~ /definitionstop/) and ($length==0)) # definitionstop at the beginning of the overlap
{
next;
}
if (($s !~ /definitionstop/) and ($i==$array_length)) # reach the end of the overlap
{
$overlap_string .= "$s ";
$num = $overlaps->{$overlap};
$value = $num * ($length**2);
$score += $value;
$length = 0;
if (defined $debugfile)
{
print DEBUG "$overlap_string $value\n";
}
$overlap_string = "";
}
}
}
else
{
my @array = split/\s+/, $overlap;
$length = $#array + 1;
$num = $overlaps->{$overlap};
$value = $num * ($length**2);
$score += $value;
if (defined $debugfile)
{
print DEBUG "$overlap $value\n";
}
}
}
return $score;
}
sub findCompoundWord
{
my $def = shift;
my $ref_complist = shift;
my $new_def = "";
my @words = split(' ', $def);
my $size_line = @words;
for (my $i=0; $i<$size_line; $i++)
{
my $w = $words[$i];
my $flag_print_w = 0;
my $flag_comp = 0;
if(defined $ref_complist->{$w})
{
# get the compound list start with word $w
my @comps = @{$ref_complist->{$w}};
foreach my $c (@comps)
{
#compare the rest of the compound word
my @string = split(' ', $c);
my $count = 1;
foreach my $s (@string)
{
if (($i+$count)<$size_line)
{
if ($s eq $words[$i+$count])
{
$flag_comp = 1;
$count++;
}
else
{
$flag_comp = 0;
last;
}
}
} # test one compound word start by $w
# connect the compound word
if ($flag_comp==1)
{
unshift(@string, "$w");
my $comp = join('_', @string);
$new_def .= "$comp ";
if (defined $debugfile)
{
print DEBUG "compounds: $comp\n";
}
my $skip = @string-1;
$i = $i + $skip;
last;
}
} # test all the compound word start by $w
# print out the $w if it doesn't match any compound words
if (($flag_print_w==0) and ($flag_comp==0))
{
$new_def .= "$w ";
$flag_print_w = 1;
}
} # end of defined compound word start by $w
if(!defined $ref_complist->{$w})
{
$new_def .= "$w ";
}
} # end of one definition
return $new_def;
}
1;
__END__
=head1 NAME
UMLS::Similarity::lesk - Perl module for computing semantic relatedness
of concepts in the Unified Medical Language System (UMLS) using the
method described by Banerjee and Pedersen (2002).
=head1 CITATION
@article{BanerjeeP03,
title={An Adapted Lesk Algorithm for Word Sense Disambiguation using WordNet},
author={Banerjee and Pedersen},
journal={Proceedings of the Third International Conference on Intelligent Text Processiong
and Computational Linguistics},
pages={136-145},
year={2002}
month={February}
address={Mexico City}
}
=head1 SYNOPSIS
use UMLS::Interface;
use UMLS::Similarity::lesk;
my $umls = UMLS::Interface->new();
die "Unable to create UMLS::Interface object.\n" if(!$umls);
my $lesk = UMLS::Similarity::lesk->new($umls);
die "Unable to create measure object.\n" if(!$lesk);
my $cui1 = "C0018563";
my $cui2 = "C0037303";
$ts1 = $umls->getTermList($cui1);
my $term1 = pop @{$ts1};
$ts2 = $umls->getTermList($cui2);
my $term2 = pop @{$ts2};
my $value = $lesk->getRelatedness($cui1, $cui2);
print "The similarity between $cui1 ($term1) and $cui2 ($term2) is $value\n";
=head1 DESCRIPTION
This module computes the semantic relatedness of two concepts in
the UMLS according to a method described by Banerjee and Pedersen(2002).
The relatedness measure proposed by Banerjee and Pedersen is and
adaptation of Lesk's dictionary-based word sense disambiguation algorithm.
--defraw option
This is a flag for the lesk measure. The definitions
used are 'cleaned'. If the --defraw flag is set they will not be cleaned,
and it will leave the definitions in their "raw" form.
If the --defraw and --stem option use together, the --stem option
will cancel the request for "raw" defintion which is set by
--defraw.
--dictfile option
This is a dictionary file for the vector measure. It
contains the 'definitions' of a concept which would be used in the
relatedness computation. When this option is set, for the input
pair, umls-similarity.pl first find the CUIs or terms definition in
the dictfile. If the --config option is set, umls-similarity.pl will
find the definition in dictfile and in UMLS. And then, the relatedness
is computed by the combinition of UMLS and dictfile defintions.
If the --dictfile option is not set, the definiton will only come from the UMLS
defintion by the --config option.
The input pair could be the following formats.
1. cui1/term1 cui2/term2
without --dictfile option and without --config option,
use the UMLS definition of the default config file.
2. cui1/term1 cui2/term2 --dictfile ./sample/dictfile
--dictfile option is set and without --config option,
definitions only come from dictfile.
3. cui1/term1 cui2/term2 --config ./sample/leskmeasure.config
without --dictfile option, --config option is set,
definitions only come from UMLS by the config file.
4. cui1/term1 cui2/term2 --dictfile ./sample/dictfile --config ./sample/leskmeasure.config
--dictfile option is set, --config option is set,
definitions come from dictfile and UMLS. If the associated term
for each CUI is defined in the dictfile, the associated terms'
definition are also included.
Terms in the dictionary file use the delimiter : to seperate the terms and
their definition. It allows multi terms in one concept. Please see the sample
file at /sample/dictfile
--doubledef option
This a dictionary file for the vector measure. It contains the
'definitons' of a concept which could be used in the relatedness computation.
When this option is defined, for each word in the definition, it uses the word's
definition in the doubledef file.
For example, the original defintion for 'cat' is,
cat: a feline pet
And then, the word vector for feline and pet in the doubledef file is:
feline: small to medium-sized cats, cougar cheetah
pet: cat dog bird fish
The final definition for cat is to combine the original definition for cat, and
then add the definition for feline(only add once) and pet.
cat: a feline pet small to medium-sized cats cougar cheetah cat dog bird fish
Terms in the dictionary file use the delimiter : to seperate the terms and
their definition. It has the same format with the dictfile. Please see the
sample file at /sample/dictfile. We extract the definition from
the WordNet by glossFinder. For the extraced file, we further parse
each senses of the same word and obtain a complete definition of the
word.
--compoundfile options
This is a compound word list for the vector measure. It defines
the compound words which are treated as one word in the definitions. This
must be used with the vector or lesk method.
For example, the definition for iraq and france are:
iraq : saddam hussein
france : jacques chirac
In the --compoundfile file, "saddam hussein" and "jacques chirac" are compounds:
jacques_chirac
saddam_hussein
So, the compound words in the definition could be detected:
iraq : saddam_hussein
france : jacques_chirac
The lesk method searches the overlap of iraq and france definitions and get
the lesk relatedness scores.
--config option
This is configure file for the lesk or vector measure. It defines
the relationship, source and rela relationship. When compute the relatedness
of a pair, umls-similarity.pl find the corresponding relationshps and
source by the config file.
--stoplist option
This is a word list file for the lesk measure. The words
in the file should be removed from the definition. In the stop list file,
each word is in the regular expression format. A stop word sample file
is under the samples folder which is called stoplist-nsp.regex.
--stem option
This is a flag for the lesk measure. If we the --stem flag
is set, the words of the definition are stemmed by the the Porter Stemming
algorithm.
=head1 USAGE
The semantic relatedness modules in this distribution are built as classes
that expose the following methods:
new()
getRelatedness()
=head1 TYPICAL USAGE EXAMPLES
To create an object of the lesk measure, we would have the following
lines of code in the perl program.
use UMLS::Similarity::lesk;
$measure = UMLS::Similarity::lesk->new($interface);
The reference of the initialized object is stored in the scalar
variable '$measure'. '$interface' contains an interface object that
should have been created earlier in the program (UMLS-Interface).
If the 'new' method is unable to create the object, '$measure' would
be undefined.
To find the semantic relatedness of the concept 'blood' (C0005767) and
the concept 'cell' (C0007634) using the measure, we would write
the following piece of code:
$relatedness = $measure->getRelatedness('C0005767', 'C0007634');
=head1 CONFIGURATION OPTION
The UMLS-Interface package takes a configuration file to determine
which sources and relations to use when obtaining the extended
definitions. We call the definition used by the measure, the extended
definition because this may include definitions from related concepts.
The format of the configuration file is as follows:
SABDEF :: <include|exclude> <source1, source2, ... sourceN>
RELDEF :: <include|exclude> <relation1, relation2, ... relationN>
The possible relations that can be included in RELDEF are:
1. all of the possible relations in MRREL such as PAR, CHD, ...
2. CUI which refers the concepts definition
3. ST which refers to the concepts semantic types definition
4. TERM which refers to the concepts associated terms
For example, if we wanted to use the definitions from MSH vocabulary
and we only wanted the definition of the CUI and the definitions of the
CUIs SIB relation, the configuration file would be:
SABDEF :: include MSH
RELDEF :: include CUI, SIB
Note: RELDEF takes any of MRREL relations and two special 'relations':
1. CUI which refers to the CUIs definition
2. TERM which refers to the terms associated with the CUI
If you go to the configuration file directory, there will
be example configuration files for the different runs that
you have performed.
For more information about the configuration options please
see the README.
=head1 SEE ALSO
perl(1), UMLS::Interface
perl(1), UMLS::Similarity(3)
=head1 CONTACT US
If you have any trouble installing and using UMLS-Similarity,
please contact us via the users mailing list :
umls-similarity@yahoogroups.com
You can join this group by going to:
http://tech.groups.yahoo.com/group/umls-similarity/
You may also contact us directly if you prefer :
Bridget T. McInnes: bthomson at cs.umn.edu
Ted Pedersen : tpederse at d.umn.edu
=head1 AUTHORS
Bridget T McInnes <bthomson at cs.umn.edu>
Siddharth Patwardhan <sidd at cs.utah.edu>
Serguei Pakhomov <pakh0002 at umn.edu>
Ted Pedersen <tpederse at d.umn.edu>
Ying Liu <liux0395 at umn.edu>
=head1 COPYRIGHT AND LICENSE
Copyright 2004-2011 by Bridget T McInnes, Siddharth Patwardhan,
Serguei Pakhomov, Ying Liu and Ted Pedersen
This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
=cut