BUTTONACT						(variable)

	if nonempty, should be an instruction list that will be evaluated
	whenever a mouse button is pressed.  Note that the user may have
	released the button before the instructions are evaluated.  BUTTON
	will still output which button was most recently pressed.  CLICKPOS
	will output the position of the mouse cursor at the moment the
	button was pressed; this may be different from MOUSEPOS if the
	user moves the mouse after clicking.

	Note that it's possible for the user to press a button during the
	evaluation of the instruction list.  If this would confuse your
	program, prevent it by temporarily setting BUTTONACT to the empty
	list.  One easy way to do that is the following:

		make "buttonact [button.action]

		to button.action [:buttonact []]
		... ; whatever you want the button to do
		end

