Hey, I don't really know how m4 works. That's just an idea that occurred to me. I think m4 could keep expanding macros and processing text while a syscmd() is in flight, potentially having several syscmd() executing at once. The user would have to guarantee that the syscmd() execution is "pure", that is, that one command does not depend on side effects of previous executions. All that could be enabled by some flag, like -j which reminisces Makefile.
Best regards, Bento
