TEXINPUTS: how to handle "extensions"

[K. Berry]

    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.

Using mltex?  Or using standard TeX?  If mltex, I suggest
texmf/mltex/whatever.  If standard TeX, I suggest texmf/tex/mltex/
or texmf/tex/french/ or something.

I guess I don't yet see the purpose of texmf/extensions/.
texmf/tex/foo/ works for different formats that use standard TeX
primitives, and texmf/bar/ works for different programs that use truly
new primitives.

    The question is: what is an extension? 

A ``TeX'' binary that recognizes primitives not recognized by the
original TeX.  IMO.  Various other changes could certainly make a
particular program qualify as an extension, too -- e.g., allow more than
16 math families.

    E.g., eTeX is an extension too to some extent. 

To some extent?  It *is* one, no two ways about it (unless you're
talking about compatibility mode).  In fact, it is the one I used as an
example :-).