Problem with expl3.sty

[Karl Berry]

Hi Max,

    I've managed to accidentally make a user format before by deleting the
    system format file:

True, if dynamic mktexfmt runs, it will create user formats.
I don't think that could or should be changed.

But maybe we should disable mktexfmt by default. Norbert, wdyt?

    (I've also found that LuaTeX will build per-user format files if you
    replace the one in the TL binary directory with a newer one 

I don't understand ... -k