getrexxvar() and setrexxvar()
    GetRexxVar() attempts to extract the value of a variable from a
    running ARexx script/program.  SetRexxVar() will attempt to set the
    value of a particular variable in a running ARexx script.

checkrexxmsg()
    This function checks to make sure that a rexxmsg is from arexx
    directly. Messages used by the Rexx Variable Interface (RVI) routines
    are required to be directly from ARexx.