  In this paper we present the denotational semantics for channel mobility in
the Unifying Theories of Programming (UTP) semantics framework. The basis for
the model is the UTP theory of reactive processes (precisely, the UTP semantics
for Communicating Sequential Processes (CSP)), which is slightly extended to
allow the mobility of channels: the set of actions in which a process is
authorised to participate, originally static or constant (set during the
process's definition), is now made dynamic or variable: it can change during
the process's execution. A channel is thus moved around by communicating it via
other channels and then allowing the receiving process to extend its alphabet
with the received channel. New healthiness conditions are stated to ensure an
appropriate use of mobile channels.
