TEXINPUTS: how to handle "extensions"

[Thomas Esser]

For the next teTeX release, I want to build a special "french" format
with ML extensions. The french package needs some special files for the
initex run that I want to use.

These special files should be found by frtex and frlatex, but not by
other formats.

The above case of these two "french" formats can be generalized to
the concept of "extension". What I need is a place to put such extensions
in a way that they don't conflict with other formats or other extensions.

The current approach is to put these files into
  texmf/french/base
and let frtex and frlatex search there before any of the other directories
are searched.

The disadvantage, however is that if all other search paths include
  texmf/tex//
at their and, they'll eventually find files from the one or the other
"extension" package. These files can be "incompatible" with the "normal"
files found in other directories in texmf/tex.

My suggestion is to add one new top-level directory below texmf where
all extensions can be stored, so that we would get
  texmf/extensions/<package>
Then, I could say that some special format searches
  texmf/extensions/<package>//:texmf/tex//

Using this approach we would
  - get just one more top-level directory
  - be able to add files visible only for some (and not for all) formats

WDYT?

Thomas