<s>
An	O
activity	O
in	O
Unified	B-Language
Modeling	I-Language
Language	I-Language
(	O
UML	B-Language
)	O
is	O
a	O
major	O
task	O
that	O
must	O
take	O
place	O
in	O
order	O
to	O
fulfill	O
an	O
operation	O
contract	O
.	O
</s>
<s>
Activities	O
can	O
be	O
decomposed	O
into	O
subactivities	O
,	O
until	O
at	O
the	O
bottom	O
we	O
find	O
atomic	B-General_Concept
actions	I-General_Concept
.	O
</s>
<s>
The	O
underlying	O
conception	O
of	O
an	O
activity	O
has	O
changed	O
between	O
UML	B-Language
1.5	O
and	O
UML	B-Language
2.0	O
.	O
</s>
<s>
In	O
UML	O
2.0	O
an	O
activity	O
is	O
no	O
longer	O
based	O
on	O
the	O
state-chart	O
rather	O
it	O
is	O
based	O
on	O
a	O
Petri	B-Operating_System
net	I-Operating_System
like	O
coordination	O
mechanism	O
.	O
</s>
<s>
Actions	O
in	O
turn	O
are	O
pre-defined	O
(	O
UML	B-Language
offers	O
a	O
series	O
of	O
actions	O
for	O
this	O
)	O
.	O
</s>
