<s>
Rulelog	B-Language
is	O
an	O
expressive	O
semantic	O
rule-based	B-Language
knowledge	O
representation	O
and	O
reasoning	O
(	O
KRR	O
)	O
language	O
.	O
</s>
<s>
It	O
underlies	O
knowledge	O
representation	O
languages	O
used	O
in	O
systems	O
such	O
as	O
Flora-2	B-Language
,	O
SILK	O
and	O
others	O
.	O
</s>
<s>
It	O
extends	O
well-founded	O
declarative	B-Language
logic	I-Language
programs	I-Language
with	O
features	O
for	O
higher-order	O
syntax	O
,	O
frame	O
syntax	O
,	O
defeasibility	O
,	O
general	O
quantified	O
expressions	O
both	O
in	O
the	O
bodies	O
of	O
the	O
rules	O
and	O
their	O
heads	O
,	O
user-defined	O
functions	O
,	O
and	O
restraint	O
bounded	O
rationality	O
.	O
</s>
<s>
Rulelog	B-Language
extends	O
well-founded	B-Application
semantics	I-Application
for	O
declarative	B-Language
logic	O
rules	O
with	O
features	O
for	O
higher-order	O
syntax	O
(	O
HiLog	B-Language
)	O
,	O
frame	O
syntax	O
(	O
cf	O
.	O
</s>
<s>
F-Logic	B-Language
)	O
,	O
defeasibility	O
(	O
prioritized	O
defaults	O
)	O
,	O
general	O
formulas	O
(	O
including	O
existentials	O
and	O
disjunctions	O
in	O
rule	O
heads	O
)	O
,	O
user-defined	O
functions	O
,	O
and	O
restraint	O
bounded	O
rationality	O
.	O
</s>
<s>
Overall	O
,	O
Rulelog	B-Language
combines	O
deep	O
logical/probabilistic	O
reasoning	O
with	O
natural	B-Language
language	I-Language
processing	I-Language
(	O
NLP	B-Language
)	O
,	O
and	O
complements	O
machine	O
learning	O
(	O
ML	O
)	O
.	O
</s>
<s>
Rulelog	B-Language
interoperates	O
and	O
composes	O
well	O
with	O
graph	O
databases	O
,	O
relational	O
databases	O
,	O
spreadsheets	O
,	O
XML	O
,	O
RDF/OWL	O
,	O
and	O
can	O
orchestrate	O
overall	O
hybrid	O
KRR	O
.	O
</s>
<s>
Despite	O
its	O
expressibility	O
,	O
Rulelog	B-Language
is	O
computationally	O
affordable	O
(	O
inferencing	O
is	O
worst-case	O
polynomial	O
time	O
when	O
radial	O
restraint	O
is	O
employed	O
)	O
.	O
</s>
<s>
The	O
more	O
capable	O
and	O
efficient	O
implementations	O
of	O
Rulelog	B-Language
,	O
such	O
as	O
,	O
Flora-2	B-Language
,	O
and	O
leverage	O
methods	O
from	O
Logic	B-Language
programming	I-Language
,	O
Non-monotonic	O
reasoning	O
,	O
Business	O
rules	O
,	O
the	O
Semantic	O
Web	O
,	O
and	O
Databases	O
.	O
</s>
<s>
Rulelog	B-Language
implementation	O
methods	O
(	O
in	O
systems	O
like	O
Ergo	O
,	O
Flora-2	B-Language
and	O
some	O
others	O
)	O
include	O
dependency-aware	O
smart	O
caching	O
of	O
reasoning	O
results	O
(	O
memoization	O
,	O
also	O
known	O
as	O
tabling	O
in	O
logic	B-Language
programming	I-Language
)	O
,	O
indexing	O
,	O
and	O
goal	O
reordering	O
(	O
for	O
improving	O
the	O
performance	O
of	O
joins	O
)	O
.	O
</s>
<s>
Rulelog	B-Language
builds	O
on	O
decades	O
of	O
work	O
in	O
Logic	B-Language
Programming	I-Language
and	O
Deductive	B-General_Concept
database	I-General_Concept
research	O
;	O
it	O
combines	O
several	O
different	O
extensions	O
of	O
declarative	B-Language
logic	I-Language
programs	I-Language
whose	O
language	O
and	O
implementations	O
were	O
originally	O
developed	O
by	O
a	O
number	O
of	O
different	O
researchers	O
since	O
1990	O
's	O
.	O
</s>
<s>
Many	O
of	O
Rulelog	B-Language
's	O
features	O
derive	O
from	O
earlier	O
systems	O
,	O
including	O
Flora-2	B-Language
,	O
SweetRules	O
,	O
XSB	B-Language
,	O
SWSL	O
,	O
and	O
others	O
.	O
</s>
<s>
There	O
was	O
a	O
number	O
of	O
standardization	O
efforts	O
for	O
precursors	O
of	O
Rulelog	B-Language
:	O
</s>
<s>
is	O
a	O
W3C	O
recommendation	O
,	O
which	O
is	O
intended	O
as	O
a	O
means	O
for	O
formal	O
specification	O
of	O
Web	O
logic	B-Language
languages	I-Language
such	O
as	O
Rulelog	B-Language
.	O
</s>
<s>
A	O
version	O
of	O
the	O
Rulelog	B-Language
specification	O
using	O
the	O
RIF-FLD	O
framework	O
is	O
standardized	O
by	O
RuleML	B-Language
.	O
</s>
<s>
Flora-2	B-Language
:	O
an	O
open	O
source	O
rule-based	B-Language
system	O
for	O
knowledge	O
representation	O
and	O
reasoning	O
.	O
</s>
<s>
:	O
a	O
commercial	O
implementation	O
of	O
Rulelog	B-Language
by	O
Coherent	O
Knowledge	O
,	O
which	O
includes	O
an	O
IDE	O
and	O
many	O
extensions	O
.	O
</s>
<s>
:	O
an	O
integrated	O
development	O
environment	O
for	O
Flora-2	B-Language
.	O
</s>
<s>
:	O
a	O
commercial	O
implementation	O
of	O
a	O
subset	O
of	O
Rulelog	B-Language
,	O
which	O
is	O
largely	O
based	O
on	O
F-logic	B-Language
with	O
various	O
extensions	O
.	O
</s>
<s>
XSB	B-Language
:	O
supports	O
a	O
smaller	O
subset	O
of	O
Rulelog	B-Language
's	O
features	O
,	O
but	O
a	O
number	O
of	O
other	O
systems	O
,	O
like	O
Flora-2	B-Language
and	O
Ergo	O
,	O
are	O
based	O
on	O
XSB	B-Language
.	O
</s>
