<s>
In	O
artificial	B-Application
intelligence	I-Application
,	O
reactive	B-Application
planning	I-Application
denotes	O
a	O
group	O
of	O
techniques	O
for	O
action	O
selection	O
by	O
autonomous	B-General_Concept
agents	I-General_Concept
.	O
</s>
<s>
These	O
techniques	O
differ	O
from	O
classical	B-Application
planning	I-Application
in	O
two	O
aspects	O
.	O
</s>
<s>
Although	O
the	O
term	O
reactive	B-Application
planning	I-Application
goes	O
back	O
to	O
at	O
least	O
1988	O
,	O
the	O
term	O
"	O
reactive	O
"	O
has	O
now	O
become	O
a	O
pejorative	O
used	O
as	O
an	O
antonym	O
for	O
proactive	B-Operating_System
.	O
</s>
<s>
Since	O
nearly	O
all	O
agents	B-General_Concept
using	O
reactive	B-Application
planning	I-Application
are	O
proactive	B-Operating_System
,	O
some	O
researchers	O
have	O
begun	O
referring	O
to	O
reactive	B-Application
planning	I-Application
as	O
dynamic	B-Application
planning	I-Application
.	O
</s>
<s>
These	O
rules	O
are	O
called	O
productions	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
subsumption	B-Architecture
architecture	I-Architecture
consists	O
of	O
layers	O
of	O
interconnected	O
behaviors	O
,	O
each	O
actually	O
a	O
finite	B-Architecture
state	I-Architecture
machine	I-Architecture
which	O
acts	O
in	O
response	O
to	O
an	O
appropriate	O
input	O
.	O
</s>
<s>
Other	O
systems	O
may	O
use	O
trees	B-Application
,	O
or	O
may	O
include	O
special	O
mechanisms	O
for	O
changing	O
which	O
goal	O
/	O
rule	O
subset	O
is	O
currently	O
most	O
important	O
.	O
</s>
<s>
in	O
Soar	B-Architecture
architecture	O
)	O
,	O
</s>
<s>
in	O
ACT-R	B-Language
)	O
,	O
</s>
<s>
exploiting	O
a	O
form	O
of	O
planning	B-Application
.	O
</s>
<s>
Expert	O
systems	O
often	O
use	O
other	O
simpler	O
heuristics	B-Algorithm
such	O
as	O
recency	O
for	O
selecting	O
rules	O
,	O
but	O
it	O
is	O
difficult	O
to	O
guarantee	O
good	O
behavior	O
in	O
a	O
large	O
system	O
with	O
simple	O
approaches	O
.	O
</s>
<s>
Some	O
limitations	O
of	O
this	O
kind	O
of	O
reactive	B-Application
planning	I-Application
can	O
be	O
found	O
in	O
Brom	O
(	O
2005	O
)	O
.	O
</s>
<s>
Finite	B-Architecture
state	I-Architecture
machine	I-Architecture
(	O
FSM	O
)	O
is	O
model	O
of	O
behaviour	O
of	O
a	O
system	O
.	O
</s>
<s>
Modeling	O
behaviour	O
of	O
agents	B-General_Concept
is	O
only	O
one	O
of	O
their	O
possible	O
applications	O
.	O
</s>
<s>
Reactive	O
plans	O
can	O
be	O
expressed	O
also	O
by	O
connectionist	O
networks	O
like	O
artificial	B-Architecture
neural	I-Architecture
networks	I-Architecture
or	O
free-flow	O
hierarchies	O
.	O
</s>
<s>
Typical	O
reactive	B-Application
planning	I-Application
algorithm	O
just	O
evaluates	O
if-then	O
rules	O
or	O
computes	O
the	O
state	O
of	O
a	O
connectionist	O
network	O
.	O
</s>
<s>
Rete	B-Algorithm
evaluation	I-Algorithm
:	O
with	O
a	O
proper	O
logic	O
representation	O
(	O
which	O
is	O
suitable	O
only	O
for	O
crisp	O
rules	O
)	O
,	O
the	O
rules	O
need	O
not	O
to	O
be	O
re-evaluated	O
at	O
every	O
time	O
step	O
.	O
</s>
<s>
Scripting	B-Language
languages	I-Language
:	O
Sometimes	O
,	O
the	O
rules	O
or	O
FSMs	O
are	O
directly	O
the	O
primitives	O
of	O
an	O
architecture	O
(	O
e.g.	O
</s>
<s>
in	O
Soar	B-Architecture
)	O
.	O
</s>
<s>
But	O
more	O
often	O
,	O
reactive	O
plans	O
are	O
programmed	O
in	O
a	O
scripting	B-Language
language	I-Language
,	O
where	O
the	O
rules	O
are	O
only	O
one	O
of	O
the	O
primitives	O
(	O
like	O
in	O
JAM	O
or	O
ABL	O
)	O
.	O
</s>
<s>
Steering	O
is	O
a	O
special	O
reactive	O
technique	O
used	O
in	O
navigation	O
of	O
agents	B-General_Concept
.	O
</s>
<s>
This	O
kind	O
of	O
steering	O
is	O
based	O
on	O
the	O
original	O
work	O
on	O
boids	B-Application
of	O
Craig	O
Reynolds	O
.	O
</s>
<s>
in	O
Milani	O
)	O
,	O
which	O
is	O
a	O
form	O
of	O
planning	B-Application
.	O
</s>
