Restrict points to limited area

Restrict the points A, B, and C to the lower left quadrant.

The underlying JavaScript code

var brd = JXG.JSXGraph.initBoard('jxgbox', {boundingbox:[-5,5,5,-5], axis:true});
var A = brd.create('point', [-1,-1], {name:'A'});
var B = brd.create('point', [-3, -3], {name:'B'});
var C = brd.create('point', [ -2, 0], {name:'C'});

brd.on('move', function() {
   var list = [A, B, C], i;

   brd.suspendUpdate();
   for (i = 0; i < list.length; ++i) {
       list[i].moveTo(
           [
            Math.min(0, list[i].X()), 
            Math.min(0, list[i].Y())
           ]
         );
   }
   brd.unsuspendUpdate();
 });