modify-category-entry

modify-category-entry is a built-in function in `C source code'.

(modify-category-entry CHARACTER CATEGORY &optional TABLE RESET)

Modify the category set of CHARACTER by adding CATEGORY to it.
The category is changed only for table TABLE, which defaults to
the current buffer's category table.
CHARACTER can be either a single character or a cons representing the
lower and upper ends of an inclusive character range to modify.
If optional fourth argument RESET is non-nil,
then delete CATEGORY from the category set instead of adding it.