<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
the	O
expressive	B-Language
power	I-Language
(	O
also	O
called	O
expressiveness	O
or	O
expressivity	B-Language
)	O
of	O
a	O
language	O
is	O
the	O
breadth	O
of	O
ideas	O
that	O
can	O
be	O
represented	O
and	O
communicated	O
in	O
that	O
language	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Web	B-Language
Ontology	I-Language
Language	I-Language
expression	O
language	O
profile	O
(	O
OWL2	O
EL	O
)	O
lacks	O
ideas	O
(	O
such	O
as	O
negation	O
)	O
which	O
can	O
be	O
expressed	O
in	O
OWL2	O
RL	O
(	O
rule	O
language	O
)	O
.	O
</s>
<s>
OWL2	O
EL	O
may	O
therefore	O
be	O
said	O
to	O
have	O
less	O
expressive	B-Language
power	I-Language
than	O
OWL2	O
RL	O
.	O
</s>
<s>
So	O
OWL2	O
EL	O
trades	O
some	O
expressive	B-Language
power	I-Language
for	O
more	O
efficient	O
reasoning	O
(	O
processing	O
of	O
the	O
knowledge	O
representation	O
language	O
)	O
.	O
</s>
<s>
The	O
term	O
expressive	B-Language
power	I-Language
may	O
be	O
used	O
with	O
a	O
range	O
of	O
meaning	O
.	O
</s>
<s>
The	O
notion	O
of	O
expressive	B-Language
power	I-Language
is	O
always	O
relative	O
to	O
a	O
particular	O
kind	O
of	O
thing	O
that	O
the	O
language	O
in	O
question	O
can	O
describe	O
,	O
and	O
the	O
term	O
is	O
normally	O
used	O
when	O
comparing	O
languages	O
that	O
describe	O
the	O
same	O
kind	O
of	O
things	O
,	O
or	O
at	O
least	O
comparable	O
kinds	O
of	O
things	O
.	O
</s>
<s>
The	O
design	O
of	O
languages	O
and	O
formalisms	O
involves	O
a	O
trade-off	O
between	O
expressive	B-Language
power	I-Language
and	O
analyzability	O
.	O
</s>
<s>
Formal	O
language	O
theory	O
mostly	O
studies	O
formalisms	O
to	O
describe	O
sets	O
of	O
strings	O
,	O
such	O
as	O
context-free	O
grammars	O
and	O
regular	B-Language
expressions	I-Language
.	O
</s>
<s>
each	O
grammar	O
and	O
each	O
regular	B-Language
expression	I-Language
,	O
describes	O
a	O
particular	O
set	O
of	O
strings	O
.	O
</s>
<s>
In	O
this	O
context	O
,	O
the	O
expressive	B-Language
power	I-Language
of	O
a	O
formalism	O
is	O
the	O
set	O
of	O
sets	O
of	O
strings	O
its	O
instances	O
describe	O
,	O
and	O
comparing	O
expressive	B-Language
power	I-Language
is	O
a	O
matter	O
of	O
comparing	O
these	O
sets	O
.	O
</s>
<s>
An	O
important	O
yardstick	O
for	O
describing	O
the	O
relative	O
expressive	B-Language
power	I-Language
of	O
formalisms	O
in	O
this	O
area	O
is	O
the	O
Chomsky	O
hierarchy	O
.	O
</s>
<s>
It	O
says	O
,	O
for	O
instance	O
,	O
that	O
regular	B-Language
expressions	I-Language
,	O
nondeterministic	B-General_Concept
finite	I-General_Concept
automatons	I-General_Concept
and	O
regular	O
grammars	O
have	O
equal	O
expressive	B-Language
power	I-Language
,	O
while	O
that	O
of	O
context-free	O
grammars	O
is	O
greater	O
;	O
what	O
this	O
means	O
is	O
that	O
the	O
sets	O
of	O
sets	O
of	O
strings	O
described	O
by	O
the	O
first	O
three	O
formalisms	O
are	O
equal	O
,	O
and	O
a	O
proper	O
subset	O
of	O
the	O
set	O
of	O
sets	O
of	O
strings	O
described	O
by	O
context-free	O
grammars	O
.	O
</s>
<s>
In	O
this	O
area	O
,	O
the	O
cost	O
of	O
expressive	B-Language
power	I-Language
is	O
a	O
central	O
topic	O
of	O
study	O
.	O
</s>
<s>
It	O
is	O
known	O
,	O
for	O
instance	O
,	O
that	O
deciding	O
whether	O
two	O
arbitrary	O
regular	B-Language
expressions	I-Language
describe	O
the	O
same	O
set	O
of	O
strings	O
is	O
hard	O
,	O
while	O
doing	O
the	O
same	O
for	O
arbitrary	O
context-free	O
grammars	O
is	O
completely	O
impossible	O
.	O
</s>
<s>
For	O
a	O
Turing	B-Algorithm
complete	I-Algorithm
formalism	O
,	O
such	O
as	O
arbitrary	O
formal	O
grammars	O
,	O
not	O
only	O
this	O
problem	O
,	O
but	O
every	O
nontrivial	O
property	O
regarding	O
the	O
set	O
of	O
strings	O
they	O
describe	O
is	O
undecidable	O
,	O
a	O
fact	O
known	O
as	O
Rice	O
's	O
Theorem	O
.	O
</s>
<s>
There	O
are	O
some	O
results	O
on	O
conciseness	O
as	O
well	O
;	O
for	O
instance	O
,	O
nondeterministic	O
state	O
machines	O
and	O
regular	O
grammars	O
are	O
more	O
concise	O
than	O
regular	B-Language
expressions	I-Language
,	O
in	O
the	O
sense	O
that	O
the	O
latter	O
can	O
be	O
translated	O
to	O
the	O
former	O
without	O
a	O
blowup	O
in	O
size	O
(	O
i.e.	O
</s>
<s>
Database	B-General_Concept
theory	I-General_Concept
is	O
concerned	O
,	O
among	O
other	O
things	O
,	O
with	O
database	B-Application
queries	I-Application
,	O
e.g.	O
</s>
<s>
In	O
the	O
predominant	O
relational	B-Application
database	I-Application
paradigm	O
,	O
the	O
contents	O
of	O
a	O
database	O
are	O
described	O
as	O
a	O
finite	O
set	O
of	O
finite	O
mathematical	O
relations	O
;	O
Boolean	O
queries	O
,	O
that	O
always	O
yield	O
true	O
or	O
false	O
,	O
are	O
formulated	O
in	O
first-order	O
logic	O
.	O
</s>
<s>
It	O
turns	O
out	O
that	O
first-order	O
logic	O
is	O
lacking	O
in	O
expressive	B-Language
power	I-Language
:	O
it	O
cannot	O
express	O
certain	O
types	O
of	O
Boolean	O
queries	O
,	O
e.g.	O
</s>
<s>
However	O
,	O
adding	O
expressive	B-Language
power	I-Language
must	O
be	O
done	O
with	O
care	O
:	O
it	O
must	O
still	O
remain	O
possible	O
to	O
evaluate	O
queries	O
with	O
reasonable	O
efficiency	O
,	O
which	O
is	O
not	O
the	O
case	O
,	O
e.g.	O
,	O
for	O
second-order	O
logic	O
.	O
</s>
<s>
Consequently	O
,	O
a	O
literature	O
sprang	O
up	O
in	O
which	O
many	O
query	B-Language
languages	I-Language
and	O
language	O
constructs	O
were	O
compared	O
on	O
expressive	B-Language
power	I-Language
and	O
efficiency	O
,	O
e.g.	O
</s>
<s>
various	O
versions	O
of	O
Datalog	B-Language
.	O
</s>
<s>
Similar	O
considerations	O
apply	O
for	O
query	B-Language
languages	I-Language
on	O
other	O
types	O
of	O
data	O
,	O
e.g.	O
</s>
<s>
XML	B-Language
query	I-Language
languages	I-Language
such	O
as	O
XQuery	B-Language
.	O
</s>
