The PSM$PRINT routine must be called exactly once after all user
    routines have been specified to the print symbiont. Each user
    routine is specified to the symbiont in a call to the PSM$REPLACE
    routine.

    The PSM$PRINT routine allows you to specify whether the print
    symbiont is to be single-threaded or multithreaded, and if
    multithreaded, how many streams or threads it can have. In
    addition, this routine allows you to control the maximum size
    of the output buffer.