This user-written routine is invoked by the DECTPU built-in
    procedure CALL_USER. The built-in procedure CALL_USER passes
    three parameters to this routine. These parameters are then
    passed to the appropriate part of your application to be used
    as specified. (For example, they can be used as operands in a
    calculation within a Fortran program.) Using the string routines
    provided by the Run-Time Library, your application fills in the
    stringout parameter in the call-user routine, which returns the
    stringout value to the built-in procedure CALL_USER.

    The description of the built-in procedure CALL_USER in the DEC
    Text Processing Utility Reference Manual shows an example of a
    BASIC program that is a call-user routine.