SS$_DEBUG (defined in STARLET) is a condition you can signal from
     your  program  to invoke the debugger.  Signalling SS$_DEBUG from
     your program is equivalent  to  typing  "control-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 invoke the debugger and do 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'));