add-file-local-variable-prop-line is an interactive autoloaded Lisp function in `files-x.el'.

(add-file-local-variable-prop-line VARIABLE VALUE &optional INTERACTIVE)

Add file-local VARIABLE with its VALUE to the -*- line.

This command deletes all existing settings of VARIABLE (except `mode'
and `eval') and adds a new file-local VARIABLE with VALUE to
the -*- line.

If there is no -*- line at the beginning of the current file buffer
then this function adds it.