<s>
In	O
the	O
field	O
of	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
pre-topological	B-Algorithm
order	I-Algorithm
or	O
pre-topological	O
ordering	O
of	O
a	O
directed	O
graph	O
is	O
a	O
linear	O
ordering	O
of	O
its	O
vertices	O
such	O
that	O
if	O
there	O
is	O
a	O
directed	O
path	O
from	O
vertex	O
u	O
to	O
vertex	O
v	O
and	O
v	O
comes	O
before	O
u	O
in	O
the	O
ordering	O
,	O
then	O
there	O
is	O
also	O
a	O
directed	O
path	O
from	O
vertex	O
v	O
to	O
vertex	O
u	O
.	O
</s>
<s>
If	O
the	O
graph	O
is	O
a	O
directed	O
acyclic	O
graph	O
(	O
DAG	O
)	O
,	O
topological	B-Algorithm
orderings	I-Algorithm
are	O
pre-topological	O
orderings	O
and	O
vice	O
versa	O
.	O
</s>
