Information Technology Reference
In-Depth Information
12. Special rules may change the membrane structure, for example by removing
a membrane (releasing its content in the membrane including it), or by creat-
ing membranes, or by including a membrane (with its content) inside another
membrane, or expelling a membrane (with its content) from a membrane which
includes it. These operations correspond to basic biological phenomena (dis-
solution, vesiculation, endocytosis, exocytosis). They are easily described in
terms of bracket expressions, and may be extended and specialized in many
ways.
13. Porters or carriers of objects could be used which control the object movement
according to some transport/addressing mechanisms.
14. Many levels of membrane skins could be useful for internal localizations. For
example, only objects which are at the first level could have some kind of inter-
action with external objects.
15. Membrane structures could assume relations which are different from mem-
brane containment; for example, specific channels or communication lines
could be assumed and rules changing these connections dynamically could be
postulated.
16. Hyper-rules refer to rules which involve not only multisets, but more generally
substructures. For example a rule such as
[
h
c
acts on three
membranes and implies a complex transformation where the existence of three
specific objects in the three membranes changes their containment relationship.
In general, it would be very useful to introduce the notion of P-term, realized
by a bracket expression possibly including variables of multisets or membrane
structures. In this way, some very general operations could be represented in a
very synthetic and powerful way, For example, let
X
[
h
c
[
i
a
[
j
b
→
[
j
b
[
i
a
Z
be variables of mul-
tisets, then the following rule changes radically the inclusion structure and the
content of some membranes:
,
Y
,
[
i
a
+
X
+[
j
b
+
Y
]
j
Z
]
→
[
i
b
+
Y
+[
j
a
+
X
]
j
Z
]
i
i
17. Structured objects like strings or trees could replace the simple objects of the
original membrane systems, allowing specific operations over these structures
in the transformations described by rules.
18. Input/output strategies establish where to put the multisets which encode the
input data (according to some encoding mechanism) and where outputs of a
terminating computation have to be read. However, instead of focusing on ter-
minating computations which provide some kind of results, some external rules
(outside the skin) may yield resources, by modeling the typical interactions of
cells with the environment. In this case, the behavior of the system is intended
to cope with the realization of some dynamical regimes of interest, keeping
some variables within certain intervals, while external rules satisfy some spe-
cific constraints [65].
19. Termination strategies could be defined in many ways; for example, the pres-
ence or the absence of some symbols in some membranes could denote the end
of computation, but even the absence of applicable rules can play the same role.
