<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
unbounded	B-Application
nondeterminism	I-Application
or	O
unbounded	B-Application
indeterminacy	I-Application
is	O
a	O
property	O
of	O
concurrency	B-Operating_System
by	O
which	O
the	O
amount	O
of	O
delay	O
in	O
servicing	O
a	O
request	O
can	O
become	O
unbounded	O
as	O
a	O
result	O
of	O
arbitration	O
of	O
contention	O
for	O
shared	O
resources	O
while	O
still	O
guaranteeing	O
that	O
the	O
request	O
will	O
eventually	O
be	O
serviced	O
.	O
</s>
<s>
Unbounded	B-Application
nondeterminism	I-Application
became	O
an	O
important	O
issue	O
in	O
the	O
development	O
of	O
the	O
denotational	O
semantics	O
of	O
concurrency	B-Operating_System
,	O
and	O
later	O
became	O
part	O
of	O
research	O
into	O
the	O
theoretical	O
concept	O
of	O
hypercomputation	O
.	O
</s>
<s>
Discussion	O
of	O
unbounded	B-Application
nondeterminism	I-Application
tends	O
to	O
get	O
involved	O
with	O
discussions	O
of	O
fairness	O
.	O
</s>
<s>
An	O
example	O
of	O
the	O
role	O
of	O
fair	O
or	O
unbounded	B-Application
nondeterminism	I-Application
in	O
the	O
merging	O
of	O
strings	O
was	O
given	O
by	O
William	O
D	O
.	O
Clinger	O
,	O
in	O
his	O
1981	O
thesis	O
.	O
</s>
<s>
He	O
defined	O
a	O
"	O
fair	O
merge	B-Algorithm
"	O
of	O
two	O
strings	O
to	O
be	O
a	O
third	O
string	O
in	O
which	O
each	O
character	O
of	O
each	O
string	O
must	O
occur	O
eventually	O
.	O
</s>
<s>
It	O
appears	O
that	O
a	O
fair	O
merge	B-Algorithm
cannot	O
be	O
written	O
as	O
a	O
nondeterministic	B-General_Concept
data	O
flow	O
program	O
operating	O
on	O
streams	O
.	O
</s>
<s>
Edsger	O
Dijkstra	O
argued	O
that	O
it	O
is	O
impossible	O
to	O
implement	O
systems	O
with	O
unbounded	B-Application
nondeterminism	I-Application
.	O
</s>
<s>
Nondeterministic	B-General_Concept
Turing	B-Architecture
machines	I-Architecture
have	O
only	O
bounded	O
nondeterminism	O
.	O
</s>
<s>
Now	O
the	O
set	O
of	O
initial	O
segments	O
of	O
execution	O
sequences	O
of	O
a	O
given	O
nondeterministic	B-General_Concept
program	O
,	O
starting	O
from	O
a	O
given	O
state	O
,	O
will	O
form	O
a	O
tree	O
.	O
</s>
<s>
have	O
argued	O
that	O
it	O
is	O
possible	O
for	O
an	O
unboundedly	O
nondeterministic	B-General_Concept
program	O
to	O
solve	O
the	O
halting	O
problem	O
;	O
their	O
algorithm	O
consists	O
of	O
two	O
parts	O
defined	O
as	O
follows	O
:	O
</s>
<s>
The	O
first	O
part	O
of	O
the	O
program	O
requests	O
a	O
natural	O
number	O
from	O
the	O
second	O
part	O
;	O
after	O
receiving	O
it	O
,	O
it	O
will	O
iterate	O
the	O
desired	O
Turing	B-Architecture
machine	I-Architecture
for	O
that	O
many	O
steps	O
,	O
and	O
accept	O
or	O
reject	O
according	O
to	O
whether	O
the	O
machine	O
has	O
yet	O
halted	O
.	O
</s>
<s>
Clinger	O
and	O
Carl	O
Hewitt	O
have	O
developed	O
a	O
model	O
(	O
known	O
as	O
the	O
Actor	B-Application
model	I-Application
)	O
of	O
concurrent	O
computation	O
with	O
the	O
property	O
of	O
unbounded	B-Application
nondeterminism	I-Application
built	O
in	O
[	O
Clinger	O
1981	O
;	O
;	O
;	O
]	O
;	O
this	O
allows	O
computations	O
that	O
cannot	O
be	O
implemented	O
by	O
Turing	B-Architecture
Machines	I-Architecture
,	O
as	O
seen	O
above	O
.	O
</s>
<s>
(	O
See	O
Indeterminacy	B-Application
in	I-Application
concurrent	I-Application
computation	I-Application
.	O
)	O
</s>
<s>
Hewitt	O
justified	O
his	O
use	O
of	O
unbounded	B-Application
nondeterminism	I-Application
by	O
arguing	O
that	O
there	O
is	O
no	O
bound	O
that	O
can	O
be	O
placed	O
on	O
how	O
long	O
it	O
takes	O
a	O
computational	O
circuit	O
called	O
an	O
arbiter	O
to	O
settle	O
(	O
see	O
metastability	O
in	O
electronics	O
)	O
.	O
</s>
<s>
He	O
further	O
argued	O
that	O
Electronic	O
mail	O
enables	O
unbounded	B-Application
nondeterminism	I-Application
since	O
mail	O
can	O
be	O
stored	O
on	O
servers	O
indefinitely	O
before	O
being	O
delivered	O
,	O
and	O
that	O
data	O
links	O
to	O
servers	O
on	O
the	O
Internet	O
can	O
likewise	O
be	O
out	O
of	O
service	O
indefinitely	O
.	O
</s>
<s>
This	O
gave	O
rise	O
to	O
the	O
unbounded	B-Application
nondeterminism	I-Application
controversy	O
.	O
</s>
<s>
The	O
oldest	O
models	O
of	O
computation	O
(	O
g	O
..	O
Turing	B-Architecture
machines	I-Architecture
,	O
Post	O
productions	O
,	O
the	O
lambda	B-Language
calculus	I-Language
,	O
etc	O
.	O
)	O
</s>
<s>
The	O
global	O
state	O
approach	O
was	O
continued	O
in	O
automata	B-Application
theory	I-Application
for	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
and	O
push	B-Application
down	I-Application
stack	I-Application
machines	O
including	O
their	O
nondeterministic	B-General_Concept
versions	O
.	O
</s>
<s>
Hewitt	O
argued	O
that	O
there	O
is	O
a	O
fundamental	O
difference	O
between	O
choices	O
in	O
global	O
state	O
nondeterminism	O
and	O
the	O
arrival	O
order	O
indeterminacy	O
(	O
nondeterminism	O
)	O
of	O
his	O
Actor	B-Application
model	I-Application
.	O
</s>
