to allow for tracking of the access to specific variables
in message sequence charts.
This keyword has no semantic content, but only serves to
determine which variables should be tracked and included
in message sequence chart displays.
The value of all variables of which the declaration is prefixed
with the keyword
is updated on a separate time-line in these displays.
The use of this prefix only affects the information that Xspin
includes in message sequence charts, and the information that Spin
includes in Postscript versions of message sequence charts under