Set the secondary selection and save it to the kill ring.
The second time, kill it. CLICK should be a mouse click event.
If you have not called `mouse-start-secondary' in the clicked
buffer, activate the secondary selection and set it between point
and the click position CLICK.
Otherwise, adjust the bounds of the secondary selection.
Normally, do this by moving its beginning or end, whichever is
closer, to CLICK. But if you have selected whole words or lines,
adjust to the word or line boundary closest to CLICK instead.
If this command is called a second consecutive time with the same
CLICK position, kill the secondary selection.
(fn CLICK)