<s>
rio	B-Language
is	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
 '	O
windowing	B-Application
system	I-Application
.	O
</s>
<s>
This	O
allows	O
running	O
rio	B-Language
inside	O
of	O
another	O
window	O
manager	O
.	O
</s>
<s>
rio	B-Language
is	O
the	O
latest	O
in	O
a	O
long	O
series	O
of	O
graphical	B-Application
user	I-Application
interfaces	I-Application
developed	O
at	O
Bell	O
Labs	O
,	O
mostly	O
developed	O
by	O
Rob	O
Pike	O
,	O
the	O
concurrent	O
window	B-Application
system	I-Application
,	O
and	O
the	O
Blit	B-Protocol
(	O
which	O
predated	O
X	B-Operating_System
)	O
.	O
</s>
<s>
rio	B-Language
was	O
a	O
complete	O
rewrite	O
of	O
8½	B-Application
in	O
Alef	B-Application
.	O
</s>
<s>
As	O
Alef	B-Application
disappeared	O
due	O
to	O
being	O
too	O
difficult	O
to	O
maintain	O
given	O
the	O
number	O
of	O
people	O
working	O
on	O
Plan	B-Operating_System
9	I-Operating_System
at	O
the	O
time	O
,	O
rio	B-Language
was	O
rewritten	O
in	O
C	B-Language
.	O
This	O
was	O
done	O
using	O
the	O
Plan	B-Operating_System
9	I-Operating_System
thread	O
library	O
which	O
was	O
inspired	O
by	O
Alef	B-Application
and	O
had	O
most	O
of	O
its	O
features	O
,	O
such	O
as	O
blocking	O
channels	O
for	O
interthread	O
and	O
interprocess	O
communication	O
.	O
</s>
<s>
Another	O
important	O
change	O
,	O
due	O
more	O
to	O
the	O
environment	O
than	O
to	O
rio	B-Language
per	O
se	O
,	O
is	O
that	O
rio	B-Language
supports	O
full	O
colour	O
,	O
using	O
alpha	B-Algorithm
compositing	I-Algorithm
,	O
whereas	O
8½	B-Application
uses	O
bitblt	B-Algorithm
operations	O
.	O
</s>
<s>
Many	O
of	O
its	O
features	O
embody	O
key	O
Plan	B-Operating_System
9	I-Operating_System
design	O
concepts	O
:	O
</s>
<s>
It	O
exports	O
a	O
file	B-Application
system	I-Application
interface	O
to	O
running	O
applications	O
.	O
</s>
<s>
This	O
interface	O
is	O
the	O
same	O
rio	B-Language
receives	O
from	O
the	O
operating	O
system	O
,	O
so	O
rio	B-Language
can	O
run	O
inside	O
a	O
rio	B-Language
window	O
without	O
any	O
special	O
arrangements	O
.	O
</s>
<s>
Because	O
the	O
interface	O
uses	O
9P	B-Operating_System
,	O
rio	B-Language
is	O
network	B-General_Concept
transparent	I-General_Concept
even	O
if	O
it	O
does	O
n't	O
include	O
any	O
network-aware	O
code	O
.	O
</s>
