The XPA messaging system provides seamless communication between many
kinds of Unix programs, including X programs and Tcl/Tk programs.
It also provides an easy way for users to communicate with these
XPA-enabled programs by executing XPA client commands in the shell or
by utilizing such commands in scripts. Because XPA works both at the
programming level and the shell level, it is a powerful tool for
unifying any analysis environment: users and programmers have great
flexibility in choosing the best level or levels at which to access
XPA services, and client access can be extended or modified easily
at any time.