Re: xml-parse-file and text properties

[Richard Stallman <rms@gnu.org>]

    Usually yes, but not always.  All normal compositions
    (i.e. compositions for displaying a script in the correct
    way) are registered in composition-function-table (a
    char-table).  So, by looking up the table for each
    character, we can recover compositions (though very slow).

Interesting.

    But, it's possible to manually compose some text.

We can consider that improper use of the composition property;
we need not cater to it.

That leaves us in a peculiar in-between situation.  If discarding
these properties altered what text would be written in a file, it
would be very bad, and it would be clear we need to preserve these
properties.  But that is not the case.  Nonetheless, the text will
display wrong if subsequently reinserted in a buffer.  All in all, I
think that is enough reason for xml.el to preserve the composition
property.

So the question is how.

One first step would be to write a function that operates on a string
and discards all text properties except composition.  More generally,
all except a certain specified list of property names.

I think that should be done at the C level for speed.