<s>
is	O
a	O
window	B-Application
system	I-Application
developed	O
for	O
the	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
operating	B-General_Concept
system	I-General_Concept
by	O
Rob	O
Pike	O
.	O
</s>
<s>
Like	O
much	O
of	O
the	O
Plan	B-Operating_System
9	I-Operating_System
operating	B-General_Concept
system	I-General_Concept
,	O
many	O
operations	O
work	O
by	O
reading	O
and	O
writing	O
to	O
special	O
files	O
.	O
</s>
<s>
Because	O
of	O
the	O
limitations	O
stemming	O
from	O
its	O
unusual	O
implementation	O
,	O
has	O
been	O
completely	O
rewritten	O
into	O
its	O
successor	O
rio	B-Language
in	O
recent	O
Plan	B-Operating_System
9	I-Operating_System
versions	O
.	O
</s>
