Restrict points to limited area

The underlying JavaScript code

var brd = JXG.JSXGraph.initBoard('jxgbox', {boundingbox:[-5,5,5,-5], axis:true});
var P0 = brd.create('point', [-4,-1], {name:'', face:'[]'});
var P1 = brd.create('point', [4,-1], {name:'', face:'[]'});
var li = brd.create('line', [P0,P1]);
var A = brd.create('glider', [0,-1,li], {name:'A'});
var B = brd.create('glider', [-3,-1,li], {name:'B'});
var C = brd.create('glider', [ 3,-1,li], {name:'C'});

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

 });