sed script (/home/mypkg/inittab.sed)

   !remove
   # remove all entries from the table that are associated
   # with this package, though not necessarily just
   # with this package instance
   /^[^:]*:[^:]*:[^:]*:[^#]*#ROBOT$/d
   

   !install
   # remove any previous entry added to the table
   # for this particular change
   /^[^:]*:[^:]*:[^:]*:[^#]*#ROBOT$/d
   

   # add the needed entry at the end of the table;
   # sed(C) does not properly interpret the '$a'
   # construct if you previously deleted the last
   # line, so the command
   #        $a\
   #        rb:023456:wait:/usr/robot/bin/setup #ROBOT
   # will not work here if the file already contained
   # the modification.  Instead, you will settle for
   # inserting the entry before the last line!
   $i\
   rb:023456:wait:/usr/robot/bin/setup #ROBOT