<s>
FDR	B-Operating_System
(	O
Failures-Divergences	B-Operating_System
Refinement	I-Operating_System
)	O
and	O
subsequently	O
FDR2	B-Operating_System
,	O
FDR3	O
and	O
FDR4	O
are	O
refinement	O
checking	O
software	B-Application
tools	I-Application
,	O
designed	O
to	O
check	O
formal	O
models	O
expressed	O
in	O
Communicating	O
sequential	O
processes	O
(	O
CSP	O
)	O
.	O
</s>
<s>
FDR	B-Operating_System
is	O
often	O
described	O
as	O
a	O
model	B-Application
checker	I-Application
,	O
but	O
is	O
technically	O
a	O
refinement	O
checker	O
,	O
in	O
that	O
it	O
converts	O
two	O
CSP	O
process	O
expressions	O
into	O
Labelled	B-Application
Transition	I-Application
Systems	I-Application
(	O
LTSs	O
)	O
,	O
and	O
then	O
determines	O
whether	O
one	O
of	O
the	O
processes	O
is	O
a	O
refinement	O
of	O
the	O
other	O
within	O
some	O
specified	O
semantic	O
model	O
(	O
traces	O
,	O
failures	O
,	O
failures/divergence	O
and	O
some	O
other	O
alternatives	O
)	O
.	O
</s>
<s>
FDR2	B-Operating_System
applies	O
various	O
state-space	O
compression	O
algorithms	O
to	O
the	O
process	O
LTSs	O
in	O
order	O
to	O
reduce	O
the	O
size	O
of	O
the	O
state-space	O
that	O
must	O
be	O
explored	O
during	O
a	O
refinement	O
check	O
.	O
</s>
<s>
FDR2	B-Operating_System
has	O
gone	O
through	O
many	O
releases	O
,	O
having	O
replaced	O
the	O
earlier	O
tool	O
now	O
referred	O
to	O
as	O
FDR1	O
in	O
1995	O
.	O
</s>
<s>
FDR3	O
is	O
released	O
by	O
the	O
University	O
of	O
Oxford	O
,	O
which	O
also	O
released	O
FDR2	B-Operating_System
in	O
the	O
period	O
2008-12	O
.	O
</s>
