Re: format-mode-line

[Richard Stallman]

     > When format is nil (for mode-line-format) or t (for header-line-format),
     > the resulting string is propertized with the proper mode-line and 
header-line
     > face as the base face.

    I'll show my ignorance: I don't know what a base face is and I can't
    find a definition in the manual.

There is no term "base face".  What he means is a face to apply
by default to any characters that don't specify some other face.
So that you would get the same face actually used in the mode line.

I think adding an argument specifying a default face for format-mode-line
is a good way to do it.