lets say one wants to run sem in a way that will create 1000 processes, perhaps they don't want to create so many at a time.
Currently one has a choice between --bg (default) which will run, create the 1000 processes and then run how many in parallel one desires. one could specify --fg, but that will effectively only run one at a time.
I think there should be a mode that defaults to bg if a slot is available, otherwise blocks in fg, till a spot frees up and then backgrounds. That way one could easily limit the number of processes being created to a manageable amount, not just the number of active processes.