Alist of additional `font-lock-keywords' elements for major modes.
Each element has the form (MODE KEYWORDS . HOW).
`font-lock-set-defaults' adds the elements in the list KEYWORDS to
`font-lock-keywords' when Font Lock is turned on in major mode MODE.
If HOW is nil, KEYWORDS are added at the beginning of
`font-lock-keywords'. If it is `set', they are used to replace the
value of `font-lock-keywords'. If HOW is any other non-nil value,
they are added at the end.
This is normally set via `font-lock-add-keywords' and `font-lock-remove-keywords'.