DDF RULES
=========

1. Some DDF files are number-only (lines.ddf, sectors.ddf and
   playlist.ddf).  Some DDF files (things.ddf) have both a name and a
   number (in the format "[XXXX:1234]" -- the number is optional,
   defaulting to zero when it (and the colon) are absent.  All other
   DDF files have just a name in the square brackets.

2  For named DDF files, the name is allowed to be missing.  The engine
   will generate an internal name guaranteed to be unique.  The
   usefulness of this is questionable, it prevents the entry from
   being referenced anywhere else in DDF.  None of our own DDF files
   should omit the name.
   
3. The initial DDF files (usually read from EDGE.WAD) will not be
   treated any different to DDF files or lumps (such as the DDFXXX
   lumps found in WAD files) that are read later on.

4. When a new DDF entry has the same name as an older one (or the same
   number, for number-only files), then the older one is removed.  The
   new entry replaces it.  The new entry uses the same memory as the
   previous one, preventing any alias issues (no need to do a
   replacement scan on all potential references).

   For things.ddf, the new number in [] replaces the old number too.
   This can cause the old number to disappear.  The only implication
   is that future number lookups of the thing would either find a
   different thing or fail.

5. For things.ddf, when the number is missing, zero or negative, then
   it means that the entry is be considered as having no number at
   all.  Lookups on these numbers don't happen, so the thing will
   never be found via number lookups, only via name lookups.

6. For things.ddf, when a new DDF entry has the same number as an
   older one (but a different name), then the new entry is used for
   lookups on that number.  The older entry is not removed, and can
   still be referred to by name, but can no longer be referred to by
   that number.