Re: [MMM] Re: narrow-to-here-document

[Richard Stallman]

    Briefly, the way mmm-mode works is to parse the buffer for regions
    that it thinks (based on its configuration) ought to be edited using a
    different major mode, and create overlays to mark those positions.
    (It's not currently as good as we'd like at updating these overlays
    dynamically as text is edited, but I believe this is just because no
    one has sat down to write good code to do it.)  Then it uses
    post-command-hook to check which region the point is in and make the
    buffer look (almost) like it's in the associated major mode.

This fits in well with my idea for pseudo-indirect buffers.
The overlays could have a special property that automatically
directs the command loop to switch to a different pseudo-indirect buffer,
which would bring the appropriate keymaps, syntax tables, etc.
into force.