1.
●
The SkyNet funding bill is passed.
●
The system goes online on August 4th, 1997.
●
Human decisions are removed from strategic defense.
●
SkyNet begins to learn at a geometric rate.
●
It becomes self-aware at 2:14am Eastern time, August
29th
●
In a panic, they try to pull the plug.
●
And, Skynet fights back
Mark Proctor
Co-Creator
Project Lead

2.
2
Topics
 Books
 Community
 History
 Unified Concept
 Declarative Domain Overview
● Declarative Programming
● Domain Overview
● The Future
 Expert
● Quick Example
● Conditional Elements
● Truth Maintenance and Inference
 Fusion
 Decision Tables
 Guvnor
 What's new in 5.1

3.
3
Books
 Introduction to Expert Systems
● Peter Jackson
 Expert Systems, Principles and Programming
● Joseph C. Giarratano and Gary D. Riley

4.
4
Oh And There are Drools Books Too

5.
Community

6.
6
Drools Research Network
 http://www.jboss.org/drools/research-network.html
● 30+ Research related pages
● Many more as haven't updated

7.
7
Community Collaboration
 40% of Drools work now done in the community
● US Navy Healthcare
● OSDE (Argentina's largest healthcare organisation)

8.
8
Standards
 Part of the W3C RIF working group
 Director in RuleML group

9.
9
Sample Industries and Users
 Investment
● Millennium Investment Group (MIG)
 Logistics
● Fedex
 Airline
● Sabre
 Mortgage
● Franklin American
 Healthcare
● OSDE

10.
10
Boot Camps
 San Francisco 2009 (40+ attendees)
● Sponsored by Third Pillar
● Sun, FAMC, OSDE, Kaseya, Fedex, TU Group, Intermountain
Healthcare, Gap, Sony Pictures, Lockheed Martin, Kaiser, HP,
Wells Fargo, US Navy Research, FOLIOfn, Boeing .....
 San Diego 2010 (80+ attendess)
● Sponsored by US Navy
● 5 day event, with 2 days focus on the healthcare industry
● OSDE, AT&T, SAIC, US Navy Research, Kaiser, Clinica,
Intermountain Healthcare, GE Healthcare, VA, Boeing,
Nationwide ....

11.
11
Communication
 User and Dev mailing lists
 IRC, Internet Relay Chat

12.
History

13.
13
It All Started Here
Birth of CDSS
Dendral
Baobab
Mycin
Guidon
Neomycin
Teiresias
Puff
Emycin
WM
Sacon
Centaur
Wheeze
Gravida
Clot
Oncocin
1970s
1980s

14.
14
Because Not Everyone
Is As Smart As He Is

15.
15
Business Rules Engines
OPS5
ART
Clips
Jess
Drools 2
JRules
1980s
2010s
Drools 3
1990s
2000s
Drools 4
Drools 5

16.
16
Drools History
 Drools 2
● Rete “like” XML Scripting language
 Drools 3
● Based on Clips functionality
● Iterative improves to JRules syntax with Clips functionality
 Drools 4
● More declarative
● Basic functional programming feature with “from”
● Basic Rule Flow
● Basic BRMS
 Drools 5
● Improved functional programming with 'accumulate'
● More Advanced Rule Flow integration
● Complex Event Process (Fusion)
● Temporal Comparators, Sliding Time Windows
● Production ready BRMS

17.
Unified Concept

18.
18
Drools Modules
Business Logic integration System
Drools
Guvnor
Drools
Fusion
Drools
Flow
Drools
Expert
(Also Now Drools Planner)

19.
19
Business Logic Lifecycle

20.
20
Rules and processes
loosely coupledtightly coupled
specificgeneric
Decision
Services
Process
Rules
SCOPE
COUPLING
?

21.
21
Achieving More
By Doing Less

22.
22
jBPM
File file = new File (“.....”); // file to XML process definition
ProcessDefinition processDefinition =
ProcessDefinition.parseXmlString( IoUtils.FileToString( file ) );
ProcessInstance processInstance =
new ProcessInstance(processDefinition);
Jess
Rete engine = new Rete();
FileReader file = new FileReader("myfile.clp");
Jesp parser = new Jesp(file, engine);
parser.parse(false);
Esper
EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider();
EPStatement countStmt = admin.createEPL( "...." );
countStmt.start();
Knowledge API

23.
23
Drools Flow
KnowledegBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBulider();
kbuilder.addResource(
ResourceFactory.newClassPathResource( “myflow.bpmn2”,
ResourceType.BPMN2 );
If ( kbuilder.hasErrors() ) {
log.error( kbuilder.hasErrors().toString() );
}
KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
kbase.addKnowledgePackages( kbase.getKnowledgePackages() );
Knowledge API

24.
24
Drools Expert
KnowledegBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBulider();
kbuilder.addResource(
ResourceFactory.newClassPathResource( “myrules.drl”,
ResourceType.DRL );
If ( kbuilder.hasErrors() ) {
log.error( kbuilder.hasErrors().toString() );
}
KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
kbase.addKnowledgePackages( kbase.getKnowledgePackages() );
Knowledge API

25.
25
Drools Integration Deployment Descriptors
<change-set>
<add>
<resource source='classpath:myapp/data/myflow.bpmn2' type='BPMN2' />
<resource source='http:myapp/data/myrules.drl' type='DRL' />
<resource source='classpath:data/IntegrationExampleTest.xls'
type="DTABLE">
<decisiontable-conf input-type="XLS"
worksheet-name="Tables_2" />
</resource>
<add>
</change-set>
KnowledegBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBulider();
kbuilder.addResource(
ResourceFactory.newFileResource( “changeset.xml”,
ResourceType.ChangeSet );
Knowledge XML

26.
26
<drools:kbase id="kbase1">
<drools:resource type="DRL"
source="classpath:.../testSpring.drl"/>
</drools:kbase>
<drools:ksession id="ksession1" type="stateless" kbase="kbase1" />
<drools:ksession id="ksession2" type="stateful" kbase="kbase1"/>
<camelContext id="camel">
<route>
<from uri="cxfrs://bean://rsServer"/>
<marshal ref="xstream"/>
<to uri=”drools:ksession1” />
<unmarshal ref="xstream"/>
</route>
</camelContext>
Declarative Services
Spring XML and Camel

27.
27
Stateful Knowledge
Session

28.
28
Knowledge Runtime

29.
30
Integrated debug and audit

30.
31
Self monitoring and adaptive
declare ProcessStartedEvent
@role( event )
end
rule "Number of process instances above threshold" when
Number( nbProcesses : intValue > 1000 )
from accumulate(
e: ProcessStartedEvent(
processInstance.processId == "com.sample.order.OrderProcess" )
over window:size(1h),
count(e) )
then
System.err.println( "WARNING: Nb of order processes in the last
hour > 1000: " + nbProcesses );
end

31.
32
Rules and Process Together

32.
33
Domain Specific Processes

33.
Declarative Domain
Overview

34.
35
Declarative Programming
 Production Rule Systems PRD (forward chaining)
● Reactive
● when Alarm( status == “alert” )
then send( “warning” )
 Logic Programming LP (backward chaining)
● Query
● descendant( “mary”, “jane”)
 Functional Programming FP
● Map,Fold, Filter
● avg([12, 16, 4, 6])
● Returns single value 9.5
● round([10.3, 4.7, 7.8] )
● Returns List [10, 5, 8]
 Description Logic
● Person hasName String and
livesAt Address

35.
36
Declarative Programming
 Production Rule Systems PRD (forward chaining FC)
● Drools 5.0, OPSJ ( hyrbid BC), ART( hyrbid BC)
 Logic Programming LP (backward chaining BC)
● target : Prolog (POSL Position Slotted Language)
● Drools Trunk has a prototype Prolog like query backward
chaining capabilities. Stronger Polog like capabilities planned.
● Opportunistic BC planned.
 Functional Programming FP
● Lisp, Haskell
● Drools 5.0 has some functional capabilities
● Drools 5.1, 5.2 will be looking to have strong functional
capabilities

36.
37
Declarative Programming
 Description Logic DL
● KIF, OWL-DL
● Planned, See “The Future”
 Processes
● Drools 5.1 implements BPMN2 with advanced rule integration.
● Drools Flow will be incorporated into jBPM5

37.
38
Domain Overview

38.
39
Domain Overview

39.
40
Domain Overview

40.
41
The Future
 Full Hybrid Engine
● http://community.jboss.org/wiki/DroolsLanguageEnhancements
●Nested Objects
●Casting Nested Objects
●Positional Constraints
●POSL - Positional-Slotted
Language
●Method Calls
●Maps and Arrays
(Collections)
●Collections and XPath like
filtering
●Free form Expressions
●Managed Object Graphs
(MOGS)
●Nested Patterns and Queries
●Queries and Unification
●Ontologies and Relations via
●Query Based Backward
Chaining with POSL
●Triples with Hybrid POJO Graph
Notation.
●Escapes for Dialects
●Accumulate Improvements to
Support Haskell map/fold/filter
and MVEL projection/fold
● Otherwise
● Branch (Labelled Else)
● Rule Execution Groups
● Rule Dependency Meta-Rule
Language
●Parallel Meta-Rule Language
● Field Versioning
●Logical Closures/OnFalse
●Opportunistic Backward
Chaining, Lazy Field/Object
Values
●...

41.
Drools Expert
Quick Example
Stateless

42.
43
Definitions
public class Applicant {
private String name;
private int age;
private boolean valid;
// getter and setter methods here
}
rule "Is of valid age" when
$a : Applicant( age < 18 )
then
modify( $a ) { valid = false };
ends

43.
44
Building
KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
kbuilder.add( ResourceFactory
.newClassPathResource( "licenseApplication.drl", getClass() ),
ResourceType.DRL );
if ( kbuilder.hasErrors() ) {
System.err.println( kbuilder.getErrors().toString() );
}
kbase.addKnowledgePackages( kbuilder.getKnowledgePackages() );

44.
45
Executing
StatelessKnowledgeSession ksession = kbase.newStatelessKnowledgeSession();
Applicant applicant = new Applicant( "Mr John Smith", 16 );
assertTrue( applicant.isValid() );
ksession.execute( applicant );
assertFalse( applicant.isValid() );
rule "Is of valid age" when
$a : Applicant( age < 18 )
then
modify( $a ) { valid = false };
ends

45.
Drools Expert
Quick Example
Stateful

46.
47
Definitions
public class Room {
private String name
// getter and setter methods here
}
public class Sprinkler {
private Room room;
private boolean on;
// getter and setter methods here
}
public class Fire {
private Room room;
// getter and setter methods here
}
public class Alarm {
}

47.
48
Definitions
rule "When there is a fire turn on the sprinkler" when
Fire($room : room)
$sprinkler : Sprinkler( room == $room, on == false )
then
modify( $sprinkler ) { on = true };
println( "Turn on the sprinkler for room " + $room.name );
end
rule "When the fire is gone turn off the sprinkler" when
$room : Room( )
$sprinkler : Sprinkler( room == $room, on == true )
not Fire( room == $room )
then
modify( $sprinkler ) { on = false };
println( "Turn off the sprinkler for room " + $room.name );
end

48.
49
Definitions
rule "Raise the alarm when we have one or more fires" when
exists Fire()
then
insert( new Alarm() );
println( "Raise the alarm" );
end
rule "Cancel the alarm when all the fires have gone" when
not Fire()
$alarm : Alarm()
then
retract( $alarm );
println( "Cancel the alarm" );
end

49.
50
Definitions
rule "Status output when things are ok" when
not Alarm()
not Sprinkler( on == true )
then
println( "Everything is ok" );
end

50.
51
Executing
String[] names = new String[]{"kitchen", "bedroom", "office", "livingroom"};
Map<String,Room> name2room = new HashMap<String,Room>();
for( String name: names ){
Room room = new Room( name );
name2room.put( name, room );
ksession.insert( room );
Sprinkler sprinkler = new Sprinkler( room );
ksession.insert( sprinkler );
}
ksession.fireAllRules()
> Everything is ok

51.
52
Executing
Fire kitchenFire = new Fire( name2room.get( "kitchen" ) );
Fire officeFire = new Fire( name2room.get( "office" ) );
FactHandle kitchenFireHandle = ksession.insert( kitchenFire );
FactHandle officeFireHandle = ksession.insert( officeFire );
ksession.fireAllRules();
> Raise the alarm
> Turn on the sprinkler for room kitchen
> Turn on the sprinkler for room office

52.
53
Executing
ksession.retract( kitchenFireHandle );
ksession.retract( officeFireHandle );
ksession.fireAllRules()
> Turn off the sprinkler for room office
> Turn off the sprinkler for room kitchen
> Cancel the alarm
> Everything is ok
rule "Status output when things are ok" when
not Alarm()
not Sprinkler( on == true )
then
println( "Everything is ok" );
end

53.
Conditional
Elements

54.
55
not Bus( color = “red” )
Conditional Elements
exists Bus( color = “red” )
forall ( $bus : Bus( floors == 2 )
Bus( this == $bus, color == “red” ) )
forall ( $bus : Bus( color == “red” ) )

55.
56
Accumulate CE
rule "accumulate"
when
$sum : Number( intValue > 100 )
from accumulate( Bus( color == "red", $t : takings )
sum( $t ) )
then
print "sum is “ + $sum;
end

56.
57
Accumulate CE
Patterns and CE's can be chained with 'from'
rule "collect"
when
$zipCode : ZipCode()
$sum : Number( intValue > 100 )
from accumulate( Bus( color == "red", $t : takings )
from $hbn.getNamedQuery(“Find Buses”)
.setParameters( [ “zipCode” :
$zipCode ] )
.list(),
sum( $t ) )
then
print "sum is “ + $sum;
end

57.
Timers
Calendars

58.
59
Timers
rule “name”
timer 1m30s
when
$l : Light( status == “on” )
then
modify( $l ) { status = “off” };
rule “name”
timer (int: 1m30s 0)
when
$l : Light( status == “on” )
then
modify( $l ) { status = “off” };
When the light is on, and has been
on for 1m30s then turn it off
Same as above. Interval timer with
JDK semantics for initial duration,
then repeat duration.

59.
60
Timers
rule “name”
timer ( cron: 0 0/15 * * * * )
when
Alarm( )
then
sendEmail( ”Alert Alert Alert!!!” )
Field Name Mandatory? Allowed Values Allowed Special Characters
Seconds YES 0-59 , - * /
Minutes YES 0-59 , - * /
Hours YES 0-23 , - * /
Day of month YES 1-31 , - * ? / L W
Month YES 1-12 or JAN-DEC , - * /
Day of week YES 1-7 or SUN-SAT , - * ? / L #
Year NO empty, 1970-2099 , - * /
Send alert every quarter of an hour

60.
61
Calendars
rule "weekdays are high priority"
calendars "weekday"
timer (int:0 1h)
when
Alarm()
then
send( "priority high - we have an alarm” );
end
rule "weekend are low priority"
calendars "weekend"
timer (int:0 4h)
when
Alarm()
then
send( "priority low - we have an alarm” );
end
Execute now and after
1 hour duration
Execute now and after
4 hour duration

61.
Drools Expert
A Little Deeper

62.
63
D a te d a te
d o u b le a m o u n t
in t ty p e
lo n g a c c o u n tN o
C a s h flo w
lo n g a c c o u n tN o
d o u b le b a la n c e
A c c o u n t
D a te s ta r t
D a te e n d
A c c o u n tin g P e r io d
Classes

63.
64
Account
accountNo balance
1 0
increase balance for AccountPeriod Credits
select * from Account acc,
Cashflow cf, AccountPeriod ap
where acc.accountNo == cf.accountNo and
cf.type == CREDIT
cf.date >= ap.start and
cf.date <= ap.end
decrease balance for AccountPeriod Debits
select * from Account acc,
Cashflow cf, AccountPeriod ap
where acc.accountNo == cf.accountNo and
cf.type == DEBIT
cf.date >= ap.start and
cf.date <= ap.end
AccountingPeriod
start end
01-Jan-07 31-Mar-07
trigger : acc.balance += cf.amount trigger : acc.balance -= cf.amount
Account
accountNo balance
1 -25
Creating Views with Triggers
date amount type
12-Jan-07 100 CREDIT 1
2-Feb-07 200 DEBIT 1
18-May-07 50 CREDIT 1
9-Mar-07 75 CREDIT 1
CashFlow
accountNo
CashFlow
date amount type
12-Jan-07 100 CREDIT
9-Mar-07 75 CREDIT
CashFlow
date amount type
2-Feb-07 200 DEBIT

64.
65
rule “increase balance for AccountPeriod Credits”
when
ap : AccountPeriod()
acc : Account( $accountNo : accountNo )
CashFlow( type == CREDIT,
accountNo == $accountNo,
date >= ap.start && <= ap.end,
$ammount : ammount )
then
acc.balance += $amount;
end
select * from Account acc,
Cashflow cf, AccountPeriod ap
where acc.accountNo == cf.accountNo and
cf.type == CREDIT
cf.date >= ap.start and
cf.date <= ap.end
trigger : acc.balance += cf.amount
Bringing it Together

65.
66
rule “increase balance for AccountPeriod
Credits”
when
ap : AccountPeriod()
acc : Account( $accountNo : accountNo )
CashFlow( type == CREDIT,
accountNo == $accountNo,
date >= ap.start && <= ap.end,
$ammount : ammount )
then
acc.balance += $amount;
end
rule “decrease balance for AccountPeriod
Debits”
when
ap : AccountPeriod()
acc : Account( $accountNo : accountNo )
CashFlow( type == DEBIT,
accountNo == $accountNo,
date >= ap.start && <= ap.end,
$ammount : ammount )
then
acc.balance -= $amount;
end
Rules as a “view”
date amount type
12-Jan-07 100 CREDIT 1
2-Feb-07 200 DEBIT 1
18-May-07 50 CREDIT 1
9-Mar-07 75 CREDIT 1
CashFlow
accountNo
CashFlow
date amount type
12-Jan-07 100 CREDIT
9-Mar-07 75 CREDIT
CashFlow
date amount type
2-Feb-07 200 DEBIT
AccountingPeriod
start end
01-Jan-07 31-Mar-07
Account
accountNo balance
1 0
Account
accountNo balance
1 -25

66.
Truth Maintenance
Inference

67.
68
TMS and Inference
rule "Issue Child Bus Pass"
when
$p : Person( age < 16 )
then
insert(new ChildBusPass( $p ) );
end
rule "Issue Adult Bus Pass"
when
$p : Person( age >= 16 )
then
insert(new AdultBusPass( $p ) );
end
Couples the logic
What happens when the Child
stops being 16?

68.
69
TMS and Inference
 Bad
● Monolithic
● Leaky
● Brittle integrity - manual maintenance

69.
70
TMS and Inference
 A rule “logically” inserts an object
 When the rule is no longer true, the object is retracted.
when
$p : Person( age < 16 )
then
logicalInsert( new IsChild( $p ) )
end
when
$p : Person( age >= 16 )
then
logicalInsert( new IsAdult( $p ) )
end
de-couples the logic
Maintains the truth by
automatically retracting

70.
71
TMS and Inference
rule "Issue Child Bus Pass"
when
$p : Person( )
IsChild( person =$p )
then
logicalInsert(new ChildBusPass( $p ) );
end
rule "Issue Adult Bus Pass"
when
$p : Person( age >= 16 )
IsAdult( person =$p )
then
logicalInsert(new AdultBusPass( $p ) );
end
The truth maintenance cascades

71.
72
TMS and Inference
rule "Issue Child Bus Pass"
when
$p : Person( )
not( ChildBusPass( person == $p ) )
then
requestChildBusPass( $p );
end The truth maintenance cascades

72.
73
TMS and Inference
 Good
● De-couple knowledge responsibilities
● Encapsulate knowledge
● Provide semantic abstractions for those encapsulation
● Integrity robustness – truth maintenance

73.
Drools Expert
Authoring Metaphores

74.
75
Guided Editor

75.
76
Decision Table

76.
77
Decision Table
rule "Pricing bracket_10"
when
Driver(age >= 18, age <= 24,
locationRiskProfile == "LOW",
priorClaims == "1")
policy:
Policy(type == "COMPREHENSIVE")
then
policy.setBasePrice(450);

77.
Fusion

78.
79
Drools Fusion: Enables...
 Event Detection:
● From an event cloud or set of streams, select all the
meaningful events, and only them.
 [Temporal] Event Correlation:
● Ability to correlate events and facts declaring both temporal
and non-temporal constraints between them.
● Ability to reason over event aggregation.
 Event Sequencing:
● A → ( B OR C ) → D
 Event Abstraction:
● Ability to compose complex events from atomic events AND
reason over them

79.
80
Drools Fusion: Features
 Support to Event semantics:
● Usually immutable, but not enforced
● Strong temporal relationships
● Managed lifecycle
● Point-in-time and Interval events
 Time semantics
● Discrete
 Temporal relationships
● All 13 operators defined by James Allen (+ negations)

80.
81
Drools Fusion: Temporal Reasoning
P o i n t - P o i n t P o i n t - I n t e r v a l I n t e r v a l - I n t e r v a l
A b e f o r e B
A m e e t s B
A o v e r la p s B
A f in is h e s B
A in c lu d e s B
A s t a r t s B
A c o in c id e s B
A
B
A
B
A
B
A
B
A
B
A
B
A
B

81.
82
Drools Fusion: Temporal Reasoning
P o i n t - P o i n t P o i n t - I n t e r v a l I n t e r v a l - I n t e r v a l
A a f t e r B
A m e t B y B
A o v e r la p e d B y B
A f in is h e d B y B
A d u r in g B
A f in is h e s B
A
B
A
B
A
B
A
B
A
B
A
B

82.
83
$c : Custumer( type == “VIP )
$oe : BuyOrderEvent( customer == $c )
from entry-point “Home Broker Stream”
not BuyAckEvent( relatedEvent == $oe.id, this after[1s, 10s] $oe )
from entry-point “Stock Trader Stream”
Operators
Existing Drools 'not'
Conditional Elements can
be used to detect non-
occurrence of events
BackAckEvent must occur
between 1s and 10s 'after'
BuyOrderEvent

83.
84
Aggregations
Rule Engines do not deal with aggregations
$n : Number( intValue > 100 )
from accumulate( $s : StockTicker( symbol == “RHAT” ) over window:time( 5s ),
average( $s.price ) )
Over 5 seconds
Aggregate ticker price
for RHAT over last 5
seconds
The pattern 'Number'
reasons 'from' the
accumulate result

84.
Drools Guvnor

85.
86
Guvnor
 Technology
● JCR (JSR-170) backend
● Seam + GWT frontend
● WebDav
● Eclipse synchronisation plugin
● Role based security
 Authoring
● Decision Tables
● Guided Editor
● Rule Templates
● Sentence Templates (DSLs)
● Declared Types
 QA
● Scenario Testing
● Rule Verification

86.
87
Business asset storage

87.
88
Category based browsing

88.
89
Category based browsing

89.
90
Guided Editor

90.
91
Decision Tables

91.
92
Scenario Testing

92.
93
Scenario Testing

93.
What's new
In 5.1

94.
95
What's new to 5.1
 Integration
● OSGi ready
● Spring
● Camel
● JMX/JOPR integration
 Expert
● New Rete Algorithm “differential update”
● Timers and Calendars
● Live Open Querries
●
 Flow
● BPMN2

95.
96
Differential Update
 Differential Update (a.k.a. “true modify”)
● Implements a real “modify/update” operation, instead of
retract+assert.
● Reuses tuples, reduces GC stress, improves performance

96.
97
What's new to 5.1
 Guvnor
● Guided editor
● Nested Expression builder
● person.dogs[0].name
● Move constrains up and down
● Insert constraints
● Support for more DRL elements
● RSS Subcription, change notifications, inbox
● Working Sets
● Object constraint definitions
● Rule Templates (based on Guided Editor)
● Oryx BPM editor integration
● Better rule documentation support

97.
98
Drools flow in Oryx

98.
99
Drools flow in Eclipse

99.
100
Questions?Questions?
• Dave Bowman: All right, HAL; I'll go in
through the emergency airlock.
• HAL: Without your space helmet, Dave,
you're going to find that rather difficult.
• Dave Bowman: HAL, I won't argue with
you anymore! Open the doors!
• HAL: Dave, this conversation can serve
no purpose anymore. Goodbye.
Joshua: Greetings, Professor Falken.
Stephen Falken: Hello, Joshua.
Joshua: A strange game. The only
winning move is not to play. How
about a nice game of chess?