Re: format-mode-line

[Richard Stallman]

    > If FORMAT is `nil', that means to use `mode-line-format'

    This does not seem to make sense because if header-line-format is nil
    then (format-mode-line header-line-format) will presumably return the
    mode-line of selected window as a string.

    ...

    I suggest changing format-mode-line so that FORMAT is not optional and not
    give a special meaning to nil and t. I think format-mode-line is currently
    always used with a value for FORMAT. If this is a good idea, I offer to do
    it and post my patch here.

The argument above seems convincing, so please do.