xml fuel tank selector

Hi all again,My airplane has three tanks of fuel (left, central and right) and for my panel I want to make three independent switches that control if one of those tanks is activated or not. Each switch can have only two positions, on or off, so that if it is on, that tank will give combustible and not if it is off, but working independently of the other two tanks. I have attempted it with (A:Fuel tank selector 1,enum), but this is a enumerated variable I don't know how to program the two only positions that it should admit each switch.Thanks for your ideas,Rafael