Help on font-lock-add-keywords: APPEND is a poor name.

[Drew Adams]

Both Info and the doc string for font-lock-add-keywords name the
optional argument APPEND.  However, its possible values show that this
is a poor name.  A better name might be HOW or WHERE. The values are:

 - nil: add to the front
 - `set': replace the entire current value
 - other non-nil: append to the rear

Also, a better value than `set' for the replacement function would be
`replace'.