font-lock-keywords(variable)のdocstringより
;; font-lock.elをprependで検索した。

OVERRIDE and LAXMATCH are flags. If OVERRIDE is t, existing fontification can
be overwritten. If `keep', only parts not already fontified are highlighted.
If `prepend' or `append', existing fontification is merged with the new, in
which the new or existing fontification, respectively, takes precedence.
If LAXMATCH is non-nil, that means don't signal an error if there is
no match for SUBEXP in MATCHER.