SS$_DEBUG (defined in STARLET) is a condition you can signal from
    your program to start the debugger. Signalling SS$_DEBUG from
    your program is equivalent to entering Ctrl/Y followed by DEBUG
    at that point.

    You can pass commands to the debugger at the time you signal it
    with SS$_DEBUG. For example, to start the debugger and issue a
    SHOW CALLS command at a given point in your program, you could
    put the following into your program (this example is coded in
    BLISS):

      SIGNAL(SS$_DEBUG, 1,
         UPLIT BYTE(%ASCIC 'SHOW CALLS'));