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Re: Comments on OWL

From: Jim Hendler <hendler@cs.umd.edu>
Date: Sat, 9 Nov 2002 19:25:22 -0500
Message-Id: <p0511170db9f3568b3e24@[]>
To: rector@man.ac.uk, public-webont-comments@w3.org
At 6:56 PM +0000 11/8/02, Alan Rector wrote:
>Jim, Ian, and Colleagues
>I've debated jumping in for some time, but preparing and
>delivering several DAML+OIL/OWL tutorials and working
>with several groups trying to come to grips with it, convince
>me that both points are valid.  I know the issue of implied
>restrictions on defined vs primitive concepts is in some
>ways a tools issue, but I think building the abstract syntax
>in a way that so obviously causes confusion is a mistake.
>It is likely to mean that many tools builders perpetuate the problem.
>As for checks - many of us need them in many situations in which I
>don't want to pay for them in global overheads and in which
>we want to see violations transparently and not as unexpected

Alan - thanks for the comments, we will be reading them and 
responding.  In particular, editors of our various documents will 
look to see how the comments affect them -- also, you should look at 
our most recent documents (several updates and new documents have 
just become available) - some of your issues have already been 
addressed in the new semantic design and in the distinguishing of an 
OWL DL subset of the language from the OWL Full functionality.
   - Jim H.
  p.s. I am taking the liberty of reincluding your message below in a 
format that more mailers may be able to take advantage of.

Suggestions for OWL
Alan Rector
October 2002
0. Introduction
It is very late to be suggesting significant changes to the OWL 
standard.  However, after learning, using, teaching, (and blundering 
with) OWL, and the workshops and discussions around EKAW, I am 
concerned that OWL will not achieve its goals because::
… Reasoners for OWL may scale poorly because make stronger inferences 
than required or even desirable. 
… Its current proposed handling of partial and complete definitions 
makes the semantics context dependent  and requires analogous 
information to be expressed with radically different constructs for 
defined and primitive classes  This follows usual description logic 
syntax but is poor human engineering which experience has shown to be 
confusing to potential authors and users.
There is a danger that constructs may be added to OWL to address 
these and other concerns which compromise its core semantics. I 
believe these problems can be addressed without changing the core 
semantics of OWL while delivering more functionality, at less 
computational cost, in a more standard way.
The proposals are to make two sorts of changes:
1) To distinguish between 'checks' and 'properties', where checks do 
not affect inference but raise exceptions.  In particular to make it 
the default that domain and range constraints are 'checks'.
2) To modify the syntax to make the distinction between primitives 
and defined concepts clearer and to make it easier to add axioms  to 
defined concepts , so that the syntax reflects users' intuitions from 
systems developed outside the DL community while still retaining the 
core DL semantics.
Each is discussed in turn.
1.  'checks' and 'properties'
1.1 Motivation
a)	Violations of range/domain constraints are likely to be user 
errors or indicate that information is incomplete. 
b) Checks are computationally cheap; the current classification 
behaviour has a high global overhead. Users are likely to want to 
specify checks, especially on cardinality, which would be absurdly 
expensive to check if treated universal constraints to used in 
testing for satisfiability.
Range checks are currently implemented as axioms.   Builders of 
classifiers have found these difficult to implement efficiently. 
Users have found both that ontologies containing range constraints 
classify slowly and that they give unexpected results which are often 
hard to understand - either causing one (or more frequently, many) 
concepts to become unsatisfiable or because they cause them to be 
classified in unexpected ways.
Similarly, cardinality constraints often are required for input 
validation and other activities but not expected to be used in 
reasoning.  They impose high costs on a reasoner but are easily 
checked for gross violations.
Most users' requirements would be satisfied if range and domain 
constraints were implemented as simple checks, performed after 
classification, which raised exceptions if violated. 
Furthermore, violation of checks leads to error messages which are 
easy to interpret and from which it is easy to locate the fault.
There are two issues
a)	How to allow the user to indicate their intention that 
something be checked rather than implanted by inference. 
b)	How to provide the checks in a standard way.  
In effect, the user needs to indicate that they are willing to accept 
incomplete reasoning in this situation in return for greater 
efficiency and transparency.  This allows users intents to be clearly 
indicated in a way that potential implementations can take account of 
A given implementer could decide whether or not to use the author's 
meta information to produce a quick but incomplete implementation or 
ignore them to produce a potentially much slower but complete 
The checks themselves might be specified either as part of the OWL 
language or, preferably, as part of a separate "OWL Query Language" 
which performed non-DL reasoning but treated checks in a uniform and 
well specified way.

1.2 Suggestion
Add two new constructs to OWL:
1.2.1 [check] [range|domain]  <remaining syntax parallel to 
range/domain constraints>
The optional [check] before the  [range|domain]  clause would 
indicate that a check should not affect classification.  After 
classification, or in response to a query, an environment would be 
expected to return a set of pairs of concepts and checks violated by 
those concepts.  The presence of a check clause would not effect 
Ideally, I would make this the default action, and the use of 
range|domain on their own either deprecated or for clarity changed to 
an alternative syntax such as:


     isDiseaseOf check range  BodyPart
     isDiseaseOf impose range BodyPar
      Hangnail restriction isDiseaseOf someValuesFrom Nail

In the current ontology a Nail is a meta device for fastening.  The 
ontology is incomplete and there are no disjoint axioms separating 
Nail(of digit) from Nail(fastening device)
Expected behaviour in variant A:
check-violations returns {violation(isDiseaseOf check-domain  BodyPart, Nail)Š}
Expected behaviour in variant B:
Nail  is classified as a BodyPart incorrectly without further 
comment.  The error only comes to light much later in the course of 
reasoning or when the ontology is further developed and disjoint 
axioms added.

1.2.2 [check] property <remaining syntax parallel to property>
The check clause would, as above,   The check [range|domain]  clause 
would indicate that a check should be performed  after classification 
or in response to a query check-variations which would return a set 
of pairs of concepts and checks violated by those concepts.  The 
presence of a check clause would not effect classification.
Motivation: There are many cases in which authors wish to impose 
checks on users of the ontology or other authors.  Doing so via 
universal constraints is computationally expensive and the reason for 
resulting behaviour is often difficult to pinpoint. Doing so via a 
post classification check is computationally cheap and pin-points the 
error precisely.
variant A)
class Foo
    restriction hasChildren at-most-15 Person
variant B)
class-def Foo
     check restriction hasChildren at-most-15 Person
Behaviour in variant A:
If there are many other properties or a complex KB,  the 
computationally effort to determine the  satisfiability of Foo or any 
of its instances grows very rapidly.  It is extremely unlikely that 
the user actually expects to have the reasoner to detect or reason 
with this information
Behaviour in variant B:
If there are explicitly more than 30 child properties to an instance of Foo,
check-violations returns {violation(hasChildren at-most-30 Person, 
Foo)Š} with minimal overheads.  It is very likely that the user 
wishes to have an input check, and may even want to allow the limit 
to be overridden.   It may miss that some set of statements about Foo 
implies greater than 30 fillers for hasChildren, but this is a rare 
inference with a high global cost.
2. Make the distinction between defined clearer primitive classes 
clearer and unify the syntax implied restrictions
2.1 Motivation
… A common error in tools is to omit 'complete' (previously 
'defined') in defined concepts. Making inclusion of either  'partial' 
or 'complete' mandatory would make the structure symmetrical and 
require a choice.  (If tool builders want to provide an option for a 
default, that should be a matter of the tools rather than the 
… In the existing syntax, the form of restrictions which are implied 
by a concept but not part of its definition -  i.e. necessary but not 
sufficient - depends on whether a class is primitive or defined. For 
primitives, necessary but not sufficient restrictions are expressed 
as part of the 'partial' description in the class statement.  For 
defined concepts, they must be expressed through separate subclass 
axioms.  This is unintuitive to users and a source of serious 
confusion when teaching the language. 
… In the case of defined classes, the information is not expressed in 
the 'frame' of the class definitions..  This is a barrier to those 
experienced in frame systems, and makes the knowledge base difficult 
to browse. 
… In the current syntax, information which is local is expressed as 
if it were global.
… The difference in expression in the current syntax makes tedious 
and obscure  the natural evolution of ontologies.  Concepts are 
commonly represented as  primitive classes when introduced and then 
transformed in later versions into defined classes as the ontology 
develops.  In our experience this amongst the most common operations 
in ontology development.  Using the current syntax, making such a 
change requires a major reformulation.  Restrictions from the class 
statement must be transformed into apparently global subclass 
axioms,  Under the proposed modification, the change is simply to 
partition the restrictions on the original primitive class between a 
necessary and sufficient definition and a set of additional necessary 
restrictions, both of which are clearly local.  This is likely to 
correspond much more closely to the author's intent. .
2.2 Suggestion
Take the current "partial"/"Complete" keywords slightly further so 
that they introduce independent subclauses.  I would suggest changing 
the keywords for clarity from "complete" to "definition" and from 
"partial" to "implies" ("implies" is not ideal because the use is 
narrower than that in the usual Lisp syntax for DLs but does not 
conflict, insofar as I can tell, with other usages in OWL. 
Alternative suggestions welcome.)
1. Change the syntax of the so that class can be followed by either 
or both of two subclauses:

	defininedAs <description1>Š<description>
	implies <description1>Š<description>

A Class statement without a defininedAs clause introduces a "primitive" .
A Class statement with both a defininedAs and a implies clause is 
treated as if the implies clause were an axiom subclass(classname, 
A note might be added that restrictions  in the implies clause of 
defined classes should be kept simple.

A)  Using current syntax
Defining malePerson as a primitive
class MalePerson
       partial (
                 restriction hasSex someValueFrom male
                 restriction hasGenitalia  someValueFrom maleGenitalia )
Defining malePerson as defined.
class MalePerson
       complete (
                 restriction hasSex someValueFrom male)
and separately the global statement
axiom subclass(MalePerson,  restriction hasGenitalia someValueFrom 

B) Using suggested syntax.

Declaring  malePerson as a primitive
class MalePerson
       implies (
                 restriction hasSex someValueFrom male
                 restriction hasGenitalia  someValueFrom maleGenitalia )
Declaring malePerson as defined.
class MalePerson
       definition (
                 restriction hasSex someValueFrom male)
                 restriction hasGenitalia someValueFrom maleGenitalia)

Note that in A), using the existing syntax, there is a major 
difference between the two constructs resulting in from the evolution 
of the ontology.   In the second, defined, case, the notion of 
subclass in this context is likely to be  is unintuitive to users. 
In B) The information  is held together in a parallel structure. 
Even worse, consider the following..  Try explaining to users why the 
isPartOf restrictions have to be handled differently in i) and ii) 

A - current syntax)
class Hand
    partial (Hand
                restriction isPartOf UpperExtremity)

class SkinOfHand
    complete (Skin
                     restriction isPartOf someValueFrom Hand)

axiom subclass(SkinOfHand, restriction isPartOf someValueFrom 

B - Proposed syntax)
class Hand
     implies (Hand
                         restriction isPartOf someValueFrom UpperExtremity)

class SkinOfHand
     definition (Skin
                       restriction isPartOf Hand)
     implies (
                    restriction isPartOf someValueFrom SkinOfUpperExtremity)
2.3 Final note
The current syntax is a legacy of the standard Lisp syntax for DLs. 
Its flaws were  less important in early DLs in which it was 
impossible to add additional necessary restrictions to defined 
concepts.  In modern DLs where both sets of  necessary and sufficient 
conditions and individually necessary conditions can be applied to 
the same concept, it causes serious confusion and should not be 
perpetuated.   Whether a set of restrictions is necessary and 
sufficient or just necessary should be uniform and indicated by the 
syntax of the restrictions, not the syntax declaring the class.

Professor James Hendler				  hendler@cs.umd.edu
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
Univ of Maryland, College Park, MD 20742	  240-731-3822 (Cell)
Received on Saturday, 9 November 2002 19:25:33 UTC

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