Opposite angles in a cyclic quadrilateral are supplementary

						var board = JXG.JSXGraph.initBoard('box1', {
boundingbox: [-10, 10, 10, -10],showNavigation:false, showcopyright: false, axis: false,
});

var p1 = board.create('point', [0, 2.0],{name:'O',face:'',label:{offset:[-10,5]}});
var c1 = board.create('circle', [p1, 6],);
var g1 = board.create('glider', [-1.0, 4, c1],{face:'',label:{offset:[-10,10]}});

var g2 = board.create('glider', [1.0, 4, c1],{face:''});

var g3 = board.create('glider', [-1.2, 1, c1],{face:'',label:{offset:[-15,-5]}});
var g4 = board.create('glider', [1.0, 1, c1],{face:'',label:{offset:[5,-5]}});

var l1 = board.create('segment', [g1, g2]);
var l2 = board.create('segment', [g2, g4],);
var l3 = board.create('segment', [g3, g4]);
var l4 = board.create('segment', [g1, g3]);

var a1 = board.create('angle', [g3, g1, g2],{radius:1,color:'blue'});
var a2 = board.create('angle', [g2, g4, g3],{radius:1,color:'blue'});

var a3 = board.create('angle', [g4, g3, g1],{radius:1});
var a4 = board.create('angle', [g1, g2, g4],{radius:1});

var l5 = board.create('segment', [p1, g3],);
var l6 = board.create('segment', [p1, g2],);