<s>
Darwin	B-General_Concept
is	O
an	O
architecture	B-Architecture
description	I-Architecture
language	I-Architecture
(	O
ADL	O
)	O
.	O
</s>
<s>
It	O
can	O
be	O
used	O
in	O
a	O
software	B-General_Concept
engineering	I-General_Concept
context	O
to	O
describe	O
the	O
organisation	O
of	O
a	O
piece	O
of	O
software	O
in	O
terms	O
of	O
components	O
,	O
their	O
interfaces	O
and	O
the	O
bindings	O
between	O
components	O
.	O
</s>
<s>
Darwin	B-General_Concept
encourages	O
a	O
component	O
-	O
or	O
object-based	O
approach	O
to	O
program	O
structuring	O
in	O
which	O
the	O
unit	O
of	O
structure	O
(	O
the	O
component	O
)	O
hides	O
its	O
behaviour	O
behind	O
a	O
well-defined	O
interface	O
.	O
</s>
<s>
Darwin	B-General_Concept
considers	O
such	O
compositions	O
also	O
to	O
be	O
types	O
and	O
hence	O
encourages	O
hierarchical	O
composition	O
.	O
</s>
<s>
The	O
general	O
form	O
of	O
a	O
Darwin	B-General_Concept
program	O
is	O
therefore	O
the	O
tree	O
in	O
which	O
the	O
root	O
and	O
all	O
intermediate	O
nodes	O
are	O
composite	O
components	O
;	O
the	O
leaves	O
are	O
primitive	O
components	O
encapsulating	O
behavioural	O
as	O
opposed	O
to	O
structural	O
aspects	O
.	O
</s>
