Triangle axiom - affine space

Axiom (A2) "triangle rule" for affine spaces.

The underlying JavaScript code

JXG.Options.label.autoPosition = true;
JXG.Options.text.useMathJax = true;
JXG.Options.text.fontSize = 20;

var board = JXG.JSXGraph.initBoard("jxgbox", {boundingbox: [-5,5,5,-5], showCopyright:false, showNavigation:false});

var p = board.create('point', [-3,-3], {name:'p'});
var q = board.create('point', [0, 2], {name:'q'});
var r = board.create('point', [3, -3], {name:'r'});

var pq = board.create('arrow', [p, q], {withLabel: true, name:'\\(\\vec{pq}\\)', label:{position: 'top', parse: false}, lastArrow: {type: 4, size: 8}});
var qr = board.create('arrow', [q, r], {withLabel: true, name:'\\(\\vec{qr}\\)', label:{position: 'top'}, lastArrow: {type: 4, size: 8}});
var pr = board.create('arrow', [p, r], {withLabel: true, name:'\\(\\vec{pr}\\)', label:{position: 'top'}, lastArrow: {type: 4, size: 8}});