Tim Bunce wrote:

> That's going to cause pain when people using older parsers try to read
> docs written for newer ones. Would a loud warning plus some best-efforts
> fail-safe parsing be possible?

Indeed. And that's a important use-case.

But best-effort is difficult when you're talking about future-compatibility
of core constructs, which these are supposed to be. I guess best-effort
for uppercase (semantic) mark-up is just to map:

    =begin UNKNOWN
    mumble mumble mumble
    =end UNKNOWN


    =head1 UNKNOWN

    =begin para
    mumble mumble mumble
    =end para

But it's harder to see how to cope with unknown all-lower directives:

    =begin frobnication
    =end frobnication

    =for franistat


Especially the last of those, since it might be either an abbreviated
block or a pure directive. I suspect that these should either still be
fatal, or they should warn-and-ignore.


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