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context-guarded prefix
and it has the form
?.
. Such a process waits until the environment

satisfies the context expression
κ
, then performs the capability
M
and continues like the

process
P
. The process learns about its context (i.e. its environment) by evaluating the guard.

The use of context-guarded capabilities is one of the two main mechanisms for context

acquisition in CCA (the second mechanism for context acquisition is the call to a process

abstraction as discussed below). The syntax and the semantics of context expressions are

given below. We let
M.P
denote the process
True
?
M.P
, where
True
is a context expression

satisfied by all context.

,, ∷=

Process

0

inactivity

P|Q

parallel composition

(
υn
)
P

name restriction

n
[
P
]

ambient

!P

repliaction

κ!M.P

context-guarder action

x⊳(y).P

process abstraction

∷=

Locations

↑

any parent

n↑

parent n

↓

any child

n↓

child n

∷

any sibling

n∷

sibling n

ϵ

locally

∷=

Capabilities

del
n

delete n

in
n

move in n

out

move out

αx
〈
z
〉

process call

α
(
y
)

input

α〈y〉

output

Table 1. Syntax of CCA processes and capabilities

A process abstraction
⊳
(
)
.
denotes the linking of the name
x
to the process
P
where
is

a list of
formal parameters
. This linking is local to the ambient where the process abstraction is

defined. So a name
x
can be linked to a process
P
in one ambient and to a diferent process
Q

in another ambient. A call to a process abstraction named
x
is done by a capability of the

form
〈
̃
〉
where
specifies the location where the process abstraction is defined and
̃
is

the list of
actual parameters
. There must be as many actual parameters as there are formal

parameters to the process abstraction being called. The location
can be
′↑′
for any parent,

′↑′
for a specifc parent
n
,
′↓′
for any child,
′↓′
for a specific child
n
,
′∷′
for any sibling,

′∷
for a specific sibling
n
, or
(empty string) for the calling ambient itself. A process call