<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
pebble	B-Application
automaton	I-Application
is	O
any	O
variant	O
of	O
an	O
automaton	O
which	O
augments	O
the	O
original	O
model	O
with	O
a	O
finite	O
number	O
of	O
"	O
pebbles	O
"	O
that	O
may	O
be	O
used	O
to	O
mark	O
tape	O
positions	O
.	O
</s>
<s>
A	O
tree-walking	B-Application
automaton	I-Application
with	O
nested	O
pebbles	O
is	O
a	O
tree-walking	B-Application
automaton	I-Application
with	O
an	O
additional	O
finite	O
set	O
of	O
fixed	O
size	O
containing	O
pebbles	O
,	O
identified	O
with	O
.	O
</s>
<s>
Without	O
this	O
restriction	O
,	O
the	O
automaton	O
has	O
undecidable	O
emptiness	O
and	O
expressive	O
power	O
beyond	O
regular	O
tree	B-Application
languages	I-Application
.	O
</s>
