implementing a mode in one self-contained definition.
@defmac define-minor-mode mode doc [init-value [lighter [keymap]]] keyword-args@dots{} body@dots{}
-@tindex define-minor-mode
This macro defines a new minor mode whose name is @var{mode} (a
symbol). It defines a command named @var{mode} to toggle the minor
mode, with @var{doc} as its documentation string. It also defines a
feature works just like the mode-line feature, except that it's
controlled by different variables.
-@tindex header-line-format
@defvar header-line-format
This variable, local in every buffer, specifies how to display the
header line, for windows displaying the buffer. The format of the value
is the same as for @code{mode-line-format} (@pxref{Mode Line Data}).
@end defvar
-@tindex default-header-line-format
@defvar default-header-line-format
This variable holds the default @code{header-line-format} for buffers
that do not override it. This is the same as @code{(default-value