Extracting a DTD from multiple files?

[Vika Zafrin]

Hello, group.

I am working with some texts, literary and otherwise, marking them up as
I go, without a set DTD. I do not plan to have a DTD in place at all,
at least not for the encoding part of the exercise. What I would like is
for oXygen (which I use on Mac OS X) to "remember" the elements,
attributes and values I've used so far in this project. I tried using
the "project" feature, but to no avail: I can create a schema, but the
minute I add something new and tell it to "learn," it disregards the
schema and only remembers the tags/values used in the active document.

Does anyone know of a tool that would extract element-etc. information
from a _set_ of files (not just one: oXygen can do that), such that the
software I use could look at that set and add to it dynamically?

I know this is a lot to ask, so I'd be happy if such a tool (if it
exists at all) would sit quietly in the background and re-create the DTD
[or other information set] as needed, probably every five minutes or less?

I should mention that, while I've been working with XML for a short
while, and SGML for years, I don't know much about schemas or any other
way to express tags besides a DTD.

Thanks much for the help.
-Vika Zafrin