Description

   The PSM$REPLACE routine must be called each time a user service
   routine replaces a symbiont routine or is added to a set of
   symbiont routines.

   The code argument specifies the symbiont routine to be replaced.
   The routine codes that can be specified in the code argument
   are of two types: those that identify existing print symbiont
   routines and those that do not. All the routine codes are
   similar, however, in the sense that each supplies a location
   within the print symbiont execution stream where your routine can
   execute.

   By selecting a routine code that identifies an existing symbiont
   routine, you effectively disable that symbiont routine. The
   service routine that you specify might or might not perform
   the function that the disabled symbiont routine performs. If
   it does not, the net effect of the replacement is to eliminate
   that function from the list of functions performed by the print
   symbiont. Exactly what your service routine does is up to you.

   By selecting a routine code that does not identify an existing
   symbiont routine (those that identify the input-filter and
   output-filter routines), your service routine has a chance to
   execute at the location signified by the routine code. Because
   the service routine you specify to execute at this location does
   not replace another symbiont routine, your service routine is an
   addition to the set of symbiont routines.

   As mentioned, each routine code identifies a location in the
   symbiont execution stream, whether or not it identifies a
   symbiont routine.